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Tokyo Marui M870: building the ultimate airsoft shotgun

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Apologies for the bullet-point formatting. It seems I can't modify it so each list is single-spaced. If the formatting really bothers you, here's a link to this same document on my google drive:







This is a modification-oriented overview of the Tokyo Marui M870, which has been written over the course of 6 months and covers each phase of my modification and outfitting plan. Treat this writeup as a technical supplement to the more standard reviews already out there, some of which are listed in the Additional Resources section at the end of this document.


If you’re short on time, the following is the BLUF (Bottom Line Up Front) for this article:

  • This gun is okay out of the box. Somewhat better than a spring-powered tri-shot shotgun, but not significantly so. See Phase 1 notes for more details.
  • The issues plaguing earlier production runs of this gun have been largely corrected in newer batches. See Phase 2 notes for more details.
  • If you want to maximize reliability and exploit performance potential, get comfortable with taking the gun apart. This especially applies to the x-ring modification, which greatly reduces the likelihood of your gun leaking. See Phase 2 notes for more details.
  • If you plan on upgrading, use WiiTech parts. The more, the better. See Phase 2 and 5 notes for more details.
  • If you want to fine-tune your gun’s performance, converting to HPA is recommended. See Phase 4 notes for more details.
  • If you want to run ammo heavier than 0.30g, be prepared to experiment with the hopup system. See Phase 3 notes for more details.
  • If your gun ends up shooting 0.36+g bbs best, you’ll likely need to upgrade your loading system components. See Phase 5 notes for more details.
  • Fitting a real Remington 870 stock is possible, but it’ll take some work. Having a machinist perform the modification is recommended. See Phase 6 notes for more details.
  • Do not use soft or brittle bbs, like Valken. High-quality bbs like HPA and BLS minimize the likelihood and severity of jams. See the Verdict section for more details.


The contents of this article are as follows:

Inception/Viability/Selection: Background information

Phase 0: Notes gathered from various reviews about TM M870 problems and fixes

Phase 1: First impressions and stock performance

Phase 2: Disassembly, upgrade and reinforcement part selection, and installation

Phase 3: Experimenting with different hopup modifications

Phase 4: HPA conversion and performance testing

Phase 5: Internal part repair and reinforcement

Phase 6: External part selection and fitment, including converting a real Remington 870 stock to fit

Phase 7: Accessory and equipment selection

On the field/Verdict/What’s next?: After-action report, final modifications, lessons learned, and conclusion

Additional Resources/References: Helpful links and sources for information indicated by bracketed numbers


The following is the parts list for this build:

  • Base gun: Tokyo Marui M870, wood version
  • Internals
    • Amped Airsoft Integrated Grip Line (IGL) for Jack 1.1/F1/F2, screwed into ¼-28 tapped gas port
    • Blackcat mechanical box spring set
    • WiiTech 150% hammer spring, P/N 4051
    • WiiTech stainless steel valve pin, P/N 4043
    • Size 104 nitrile x-ring, modified, installed on valve pin
    • DangerWerx transfer bar
    • WiiTech aluminum loading plates, P/N 4044
    • WiiTech duster gas nozzle, P/N 4045, with nozzle shaft return spring from WiiTech top gas loading nozzle
    • WiiTech 2016 version hopup chamber, P/N 4059
    • TM hopup rubbers, shimmed with electrical tape
    • WiiTech 6.03 x 380mm extended inner barrel set, P/N 4032
  • Externals
    • Real-steel Knoxx SpecOps stock, Gen 1, machined to fit
    • Blackcat sling adapter, slot type, modified to interface with Knoxx SpecOps stock
    • TM M870 Tactical black faux bolt, P/N 870T-39
    • G&P saddle rail and shell carrier, cut down, P/N MSP-017
    • Real-steel GG&G QD T1 mount, with Dytac T1 red dot and WiiTech T1 lens protector
    • G&P railed pump, P/N MSP-002H, with Longshot Manufacturing 25-slot M-Lok rails machined to fit
    • Replica H&K vertical foregrip
    • Night Evolution M600C weapon light
    • FMA LA-5 battery box
    • Replica KAC rail covers x2, cut down, backside filed to allow passage of cables
    • WiiTech dummy magazine tube extension, P/N 4028, with coupler from G&P magazine extension
    • Real-steel Remington 870 magazine tube clamp
    • US-issue M60 sling, configured as 3-to-1 point quick adjust sling with ITW MQRB bolted to saddle rail
  • Accessories/other
    • TM shotgun shells with MAS SuperFeed springs and HexMag follower bases
    • Redline SFR regulator
    • First Strike Hero 2 48/4500 HPA tank, or dual First Strike Guerrilla 13/3000 HPA tanks
    • Amped Airsoft HPA line
    • TechT GunSav grease


If you’re interested in learning the specifics of how I transformed this rather ordinary replica into what is perhaps the ultimate airsoft shotgun, buckle up. It’s gonna be a long read.

Edited by ardrummer292
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Inception: What gave me the idea to buy a shotgun?



My primary weapons, circa 2005.


I had a couple slow days at work. Naturally, my thoughts drifted away from the job and towards airsoft. I got to thinking about how awesome it would be if TM released a semi auto Benelli M1014. That kicked off a nostalgia trip. I haven’t run an airsoft shotgun since 2006, but I distinctly remember enjoying it quite a bit. I even wrote a (pretty cringe-worthy) piece on firepower deprivation and how it relates to the airsoft “combat mindset.”[1] Seeing as how I’ve been a machine gunner for the bulk of airsoft games over the last 10 years, a change of pace certainly sounded appealing.


Viability: Does buying a shotgun make sense?



Terrain considerations are crucial when determining whether a particular type of gun will work for you. Pictured here (L to R) are what I have to negotiate: swamp and dense woods, linear open areas, and an urban complex.


Once the nostalgia faded, I started to think about what a shotgun could really do for me in-game. I wrote down every concern I could in order to better organize my thoughts. For those of you who aren’t sure if a shotgun is a good purchase, my notes might help you come to a decision.


Minimum performance parameters: I had to take a critical look at my home field in order to determine whether a shotgun was viable. Ballahack Airsoft is a challenging AO, with terrain varying from dense woods and swamp to linear expanses of open ground to an extensive built-up/urban area. Attendance is high: anywhere from 100-180 players show up on any given open play day, with head counts regularly exceeding 200 for holiday weekends and events. My conclusion was that the shotgun needed a decent amount of range, enough power to punch through brush, and a large power source (gas or battery) in order to be effective.


Fulfilling a comparable role: Real shotguns are not underpowered weapons. 12 gauge 00 buck carries 2-3 times the muzzle energy of 5.56 NATO or 7.62x39.[2,3] Accepting substandard muzzle energy from your airsoft shotgun is a disservice to the real weapon’s performance, especially in regards to brush penetration. Similarly, poor range is not realistic for a shotgun despite their portrayal in video games. Military small arms engagements frequently take place within the effective range of a shotgun using buckshot.[4,5] Realism aside, having enough reach to effectively return fire is one small way to counter the innate drawbacks of the platform.


Novelty: Shotguns might be cool, but gimmicky pieces are a waste once the novelty wears off. Once this happens, they usually languish in some forgotten pile of disused kit. One way to keep a shiny new toy gainfully employed is to ensure that it consistently performs as well as it possibly can.


Cost: The saying “you get what you pay for” has more to do with quality than utility. In order to avoid wasting money, I needed to build the shotgun into a reliable and effective fighting implement. This endeavor would undoubtedly take some expensive trial-and-error.


Technical intricacy/difficulty servicing weapon/unreliability: I’ve been exclusively using AEGs for over a decade. Learning to repair and maintain an entirely new platform would require a significant commitment, both in time and effort.


My verdict: A properly configured shotgun is a special-purpose weapon offering enhanced brush penetration characteristics and decisive close quarters firepower while being versatile enough to hold its own in longer distance engagements, if required. Using a shotgun mandates a teamwork-intensive play style and forces practice of key individual skills such as sidearm transitions, ammo management, and reflexive shooting.


Selection: How did I decide on the TM M870?


I loved my old TM M3 Super 90 and SPAS-12, but spring tri-shots honestly aren’t up to snuff as competitive primary weapons at my home field. With that category of guns dismissed, I began researching less familiar options.


There are quite a few airsoft shotguns out there, so thinning the herd took time and research. I threw out all shell-ejecting models since I’m not keen on running around picking up hulls after every engagement. I also removed all single-projectile variants (i.e. G&P) because they are more like pump-action sniper rifles than shotguns. After whittling down my list, I was left with the following choices:

  • Maruzen M1100 Defender: if I could find one

  • Modify MOD 90: may well be vaporware

  • Krytac Origin 12: technical specs and release date unknown

  • TM AA12/SGR12: not crazy about the aesthetics or the 45 degree hopup offset for increased spread

  • Walther SG9000: 88 gram CO2 cartridges aren’t cheap, plus the internal “magazine” is obnoxious

  • Marushin M500: been there done that, range is poor and reloads take far too much time

  • VFC Fabarm STF-12: unproven, liable to experience degradation over time similar to other CO2-powered guns, and pretty gross looking

  • TM KSG: reported to have less range than the M870 and more awkward to reload

  • TM M870: lots of issues listed in reviews, although most of these reviews are fairly dated

None of these guns are perfect. In the end, impressionism helped me decide. I opted for the only choice within the restrictions of my preferred kit (SAS/UKSF), which was the M870. Aside from fitting my usual bb-dressup theme, it seemed to have performance potential via extensive aftermarket support. As such, the M870 was the most logical choice.


Now you may be wondering why the Golden Eagle/JAG Scatterguns didn’t make an appearance on my list. As clones of the Tokyo Marui, I lumped them all in the same category. I opted to go for the Marui over the clones due to compatibility issues. Clone shotguns are reported to have numerous problems, not the least of which is issues with aftermarket parts. This bit of information was my primary motivation for going with TM. A second reason is a cardinal rule of mine: when buying a gas gun, if I have a choice between Tokyo Marui and an off brand clone of the same model, I bite the bullet and buy the TM. I’ve had enough non-TM gas guns irreparably fail to have learned that lesson the hard way.


Make no mistake, I’m not a blind Marui fanboy. Reports from reputable members of the community describe the TM M870 as a victim of poor design, rife with weird issues and crippling failures. Those M870 owners who haven’t experienced any problems - if they bother to say anything - are being drowned out by those whose guns are problematic. Thankfully, there is now an extensive body of technical knowledge available to fix any issues that may arise.

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Phase 0: Research


Now that I had decided to buy the gun, I wanted to know what to expect from it. This took hours of research, evaluating conflicting reports and filling in the blanks with educated guesses. Below is an abridged version of all the information I could find online. Bear in mind that some of this information dates back to the gun’s initial release in 2013, meaning that some of the teething problems mentioned within have since been solved.


Overall notes:

Most performance issues seem to be related to the loading system and gas system

  • Gas system issues can be largely avoided by using HPA

  • Loading system issues seem to stem from the tri-nozzle, loading plates and springs, and (to a lesser extent) shell choice and ammo brand

Practicality of 3-shot mode vs. 6-shot mode

  • 6-shot mode seems to use about twice as much gas as 3-shot mode

  • Muzzle velocity is nearly the same between 3-shot and 6-shot modes while using Abbey Brut Sniper gas; other gases should yield similar results

  • Ammo that yields optimum range in 3-shot mode is likely to perform poorly in 6-shot mode

  • Lighter bbs can be used to regain some range in 6-shot mode: not “light” as in 0.12g, just lighter than the bbs used for optimum performance in 3 shot mode

  • Using 2 different colors of shells is advised to distinguish between shells loaded with different weights of bbs for use in 3-shot mode or 6-shot mode

Performance on HPA per eagle258’s setup: WiiTech 380mm inner barrels, SAT valve

  • At 95 psi, 330 fps w/0.20g, 150 ft range w/0.28g

  • At 115 psi, will function (no performance data)


Notes on specific parts:

Inner barrels

  • WiiTech 6.03 380mm long inner barrels are highly desirable

    • Supposed to boost power by 20%

    • Slightly tighter groups at range, more noticeable when using 6-shot mode

    • Replaces flat nut (used to secure barrel bracket) with threaded inner barrel brace, which significantly increases ease of reassembly

    • Reduces/eliminates barrel wobble

Hop up

  • Factory fixed hop setting seems to be somewhat inconsistent from gun to gun

  • Seems to generally perform well with 0.23-0.28g

  • Hopup unit

    • Stock unit is plastic

    • WiiTech unit is aluminum, designed to apply additional pressure on top of bucking for more backspin

  • Rubbers/buckings

    • Rubbers have dual internal “mounds” in line, for hopping 2 bbs per barrel (only applicable when in 6-shot mode)

    • Feedback on SAT hopup rubbers is inconclusive: both over- and under-hop is reported


  • Stock valve yields limited power

    • WiiTech stainless steel valve pin is supposed to boost power by 12% when installed in stock valve body

  •  Feedback on SAT valve is inconclusive: some say it’s junk, others have no trouble

    • Will yield 400 fps (presumably with 0.20g) at 100 psi

    • Needs to be lubed and tightened down HARD when installed

    • Valve spring may need to be stronger in order to hold green gas-equivalent pressure; included spring can be strengthened by adding 4 coils from stock TM valve spring to SAT valve spring on SAT valve

    • End of SAT valve pin will “mushroom” over time due to repeated impacts from hammer

  • No information available regarding Pro Arms valve’s performance; the design appears to be very similar to the SAT valve, so similar performance is assumed


  • Hammer spring can be shimmed by placing an o-ring underneath it; should provide about a 30 fps boost

    • WiiTech sells a 150% hammer spring, which should provide similar results

  • Contact surfaces on hammer can be polished to smooth out action

3-/6-shot selector switch

  • Ensuring spring is set in the correct position is key to ensure consistent feeding; if selector cam is loose due to the spring becoming unseated, a random amount of bbs will load when shotgun is in either 3- or 6-shot mode


  • ACM shells don’t always feed consistently

  • No issues reported with TM shotshells

  • TM-compatible shells are about 2mm wider in diameter than real 12ga shells; real-steel polymer “pinch-type” side saddles (like Cytac/TacStar) are an extremely tight fit, real-steel aluminum side saddles (like GG&G/Mesa Tactical) outright do not work


  • Stock plastic tri-nozzle is a weak point; breakages seem to be fairly common, especially under heavy use

  • Two options for reinforcing this part of the system

    • Reinforce undamaged stock nozzle with 4mm brass tube inserts (performance and results unverified), or

    • Install reinforced aluminum nozzle from Nebula or WiiTech (WiiTech top gas loading nozzle is supposed to boost power by 14%, while WiiTech duster gas nozzle offers a 12% power boost)

    • Nebula “reinforced” plastic nozzle is prone to breakage and a waste of money 

Loading plate

  • Stock loading plate springs seem to struggle with bbs heavier than 0.28g

    •  The Blackcat mechanical box spring set includes stiffer loading plate springs

  • Reinforced loading plate can fix misfeed and rough feeding issues

    • WiiTech loading plate is reported to work better than SAT loading plate, possibly due to lighter weight

    •  Threaded loading plate pins on SAT plate will need to be loctited/JB Welded onto plate

    • Upgraded loading plates may still break; this is not necessarily a permanent fix


  • Outer barrel will end up wobbly after heavy use; WiiTech 380mm inner barrel set may help to prevent this

  • One report says that the G&P RIS forend fits much better than the WiiTech RIS forend, no further comparisons available to verify or disprove this

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Phase 1: Out of the box


The faux wood looks lovely and the metal components have a nice satin finish. The bolt cover is bright chrome, which I don’t like. I heard a worrying sliding/rattling sound upon picking it up; this was the useless shell tube storage feature, which I plan to remove at a later date. This version of the shotgun comes with an old-school red shotshell instead of the newer green or white shells. It also features a bead sight instead of the ghost ring sights on the Tactical model. The bead sight pedestal is secured to the outer barrel with double-sided tape, which you may want to reinforce with superglue or JB Weld.


The force required to rack the gun is significantly less than a spring tri-shot, but still stiffer than a real shotgun. The sound of racking the pump is quite intimidating. The sound of firing the shotgun isn’t nearly as impressive. CLA-CLACK… puf. It’s a toy gun, so I won’t harp on that point too much.


Chrono results: An Xcortech X3200 Mk3 chrono was used. Ambient temperature was 66 F, so about 19 C. The gas tank was completely drained before switching gases. The gas tank sat in my pocket for 10 minutes after each fill in order to warm up. My chrono seemed to reliably read 3 bbs passing through simultaneously, so I didn’t bother trying to load a single bb for these tests. Rounds were fired slowly, with about 15 seconds elapsed between each shot.


HFC134a (with added silicon oil), 0.20g bbs, 10 shots:

Average: 212.9 fps/0.421 J

Muzzle velocity standard deviation: 3.3 fps


HFC134a (with added silicon oil), 0.30g bbs, 10 shots:

Average: 198.2 fps/0.547 J

Muzzle velocity standard deviation: 9.2 fps

Joule creep: approximately 30.0% more energy with 0.30g than 0.20g using duster gas


Propane (with added silicon oil), 0.20g bbs, 10 shots:

Average: 314.6 fps/0.919 J

Muzzle velocity standard deviation: 12.9 fps


Propane (with added silicon oil), 0.30g bbs, 10 shots:

Average: 270.5 fps/1.020 J

Muzzle velocity standard deviation: 6.4 fps

Joule creep: approximately 10.9% more energy with 0.30g than 0.20g using propane


Ammo selection and range: using stock parts. Bear in mind that there seems to be inconsistency in hopup performance from batch to batch. As such, your results will probably vary from what I have listed below.

3-shot mode, HFC134a: 0.25g bbs work best, approximate range 110 ft

6-shot mode, HFC134a: 0.20g bbs work best, approximate range 80 ft

3-shot mode, propane: 0.28g bbs work best, approximate range 130 ft.

6-shot mode, propane: 0.20g bbs work best, approximate range 100 ft


My initial impressions are not favorable. On propane, this gun performs like a spring tri-shot with more power and less consistency. On duster gas, it’s as anemic as a spring tri-shot with none of the inherent accuracy. This lackluster performance may be partially due to the inside of the barrels being coated in grease from the factory. I’ll give it a thorough cleaning when I strip it down for upgrades later. If you’d like a more detailed look at the stock performance, KhanSeb put together an excellent video. Links to the relevant sections below:

Chrono results: https://youtu.be/dwA1oIdLXMM?t=880

Range testing: https://youtu.be/dwA1oIdLXMM?t=120

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Phase 2: Serviceability and upgrades


A gun I can’t repair myself is a gun that I won’t often use. I needed to know how the shotgun goes together and how it works, which led me to watch basically every disassembly and troubleshooting video available. Without this prior knowledge, I would’ve been much more apprehensive about buying it, let alone tearing it down.


I assumed the stock performance wasn’t going to be up to my standards, so I placed an order with WiiTech around the same time I ordered the shotgun itself. The package containing the upgrade parts arrived at my doorstep in 4 days; remarkable, considering it traveled from Hong Kong to Virginia via standard tracked shipping. I ordered the following components:


Stainless steel valve pin, P/N 4043

2016 version hopup chamber, P/N 4059

Top gas loading nozzle, P/N 4045T

6.03 x 380mm extended inner barrel set, P/N 4032


WiiTech recommends that all the above parts are installed in conjunction to yield optimum performance. The predicted muzzle energy increase per WiiTech’s website is 46% for the recommended parts, assuming energy gains “stack” instead of influencing each other. If WiiTech’s estimates are correct, the shotgun will end up shooting 1.49 J using propane and 0.30g bbs, which is right at the limit for my home field. I had reservations about the accuracy of WiiTech’s figures, so I also ordered a 150% hammer spring (P/N 4051). If WiiTech’s estimates are incorrect, I can use the spring to make up the difference.


In addition to the upgrade parts above, I ordered a packet of size 104 (2.84mm ID x 2.62mm C/S) nitrile X-rings. These will be ground into a conical shape and installed on the valve pin. I got this idea from a couple troubleshooting videos which showed this modification as a permanent output valve leak fix.[6] I haven’t had any leaks as of yet, so this is more of a preventive measure than an upgrade.



Stock TM valve (top) vs SAT valve (bottom).


I considered purchasing a SAT output valve in addition to the WiiTech parts listed above, but decided against it. I was concerned that the WiiTech valve pin wouldn’t fit in the SAT valve body, which was worrying since the SAT valve pin is known to deform under heavy use. It turns out that my concerns about incompatibility were likely justified. As you can see in the picture above, the SAT valve pin is a significantly different design than the Marui or WiiTech. This peculiarity is in order to fit a support ring, which prevents the SAT valve pin from going off-axis and causing leaks. I found that the x-ring modification mentioned earlier uses a different approach to fix the same leaking problem, and does so admirably at a fraction of the cost.[7]


Enough background, let’s strip it down! The following is a straightforward disassembly writeup from Arnie’s Airsoft user p.phresh:



Or, if wall-o-text isn’t your thing, I assembled a playlist of the most helpful videos I could find. They are in order, from disassembly to troubleshooting to reassembly. Available here:



My notes from disassembly, upgrade part installation, and reassembly are as follows:

  1. Don’t attempt to service this gun if you don’t have a good chunk of time to dedicate to it. It took me 10 hours to disassemble, upgrade, reassemble, and test it. That said, I was methodical and took a half dozen short breaks to keep myself alert and thinking clearly.

  2. LaZouche Custom’s videos are fantastic. They were my primary source of information throughout the entire process. They are included in the playlist linked above.

  3. When removing and reinstalling the trigger group, retract the forend about halfway back. After removing the two receiver pins, the trigger group should come out without any resistance.

  4. When removing the receiver shell, do so slowly. Otherwise, you’re sure to lose your transfer bar spring and ejection port cover (faux bolt) spring.



    The two recesses which house the lock washers used to secure the trigger group pins in place.


  1. Two lock washers dropped out of the receiver when I removed the mechanical box (the thing that holds all the working parts, kinda like the “gearbox” of the gun) from the receiver for the first time. These washers sit in small circular recesses on the outer face of the left half of the mechanical box shell. The purpose of the lock washers is to lock into the trigger group pins, preventing them from sliding out of the gun while in use. You can use grease to hold them in place during reassembly.



    The template I made to keep the mechanical box screws in order.


  1. I advise creating some sort of method to keep track of which screw goes in which hole. I created a template for the mechanical box screws by tracing its outline on a piece of paper. The removed screws were punched through the paper in their corresponding location on the traced outline.

  2. Some of the flat-headed (not flathead) screws, like those used to secure the transfer bar to the hammer and the 3/6-shot selector components, are picky when it comes to screwdriver bits. I found a small Phillips #00 bit works best. These small bits are usually included with cheap multi-bit screwdrivers, as are Torx/star bits which are needed for 3 of the mechanical box screws.

  3. It seems that one of the improvements TM implemented on the breacher has found its way to the wood stock version. The loading arms have a steel post, which means they are less likely to break. The gun was also thoroughly greased, which is a welcome change from early reports of M870 Tacticals coming from the factory bone dry and leaky.




    The o-rings I soaked and greased in order to beef up the airseal.

(Top L): O-ring on back end of expansion chamber assembly.

(Top R): Valve nest o-ring and valve pin modified x-ring.

(Bottom L): O-ring on front end of expansion chamber assembly and air nozzle shaft o-ring.

(Bottom R): Two air nozzle base o-rings.


  1. I performed a significant amount of preventive maintenance while inside the gun. Every o-ring I could find was carefully removed, cleaned, and soaked in Airsoft Innovations GBB (1.5wt) silicone oil for 5-10 minutes. For o-rings mounted around the outside of parts, I added 2 layers of teflon tape into the recess where the o-ring sits. The o-rings were reinstalled and then coated in TechT Gun Sav grease, which serves as a barrier to lubricant loss.



    The x-ring, before modification (L) and after sanding to the proper shape (R).


  1. The x-ring modification I mentioned earlier in this section is highly recommended. Grinding it into the proper shape required a power drill (to spin the valve pin, akin to a jury-rigged lathe), a hand file, and about 10 minutes of concentration. After installing, I can see how it will prevent leaking issues from arising in the future.

  2. There are no videos showing how to disassemble and reassemble the nozzle and hopup unit. The nozzle is simple enough that a video isn’t really necessary. The hopup unit is similar in simplicity, although the hopup system design will have long-time AEG users scratching their heads.

  3. The hopup rubbers do not overlap/wrap around the inner barrels at any point. They simply sit on top of the provided cutout on the inner barrel, which is a baffling design choice. Adding teflon tape to improve airseal will make reassembly significantly more difficult. I reinstalled the stock TM rubbers per factory methods before attempting any other modifications. For details on hopup system experimentation, see my notes in Phase 3.



(L): The block that must be removed in order to install or remove the WiiTech triple inner barrel cluster.

(R): Jamming a small flathead screwdriver under this block will keep it elevated and aligned when reinstalling the outer barrel saddle.


  1. One of the main reasons I bought the WiiTech extended inner barrel set was to replace the ever-irritating flat nut that sits inside the outer barrel. Little did I realize, the WiiTech barrel set isn’t a drop-in part. You have to remove a large threaded block inside the chamber end of the outer barrel (part of the outer barrel saddle, P/N 870T-22) in order to install the extended inner barrels. Once you’ve done that, inserting the extended barrels and reinstalling the aforementioned threaded block is fairly easy.



Proper 3/6-shot selector spring placement, with the loose end under the rectangular nozzle protrusion.


  1. Ensuring the 3/6-shot selector spring is in the correct location can be tricky. If placed incorrectly, feeding issues may result. Be sure to place the loose end of the spring below the small rectangular protrusion on the nozzle, in a position where it can provide maximum force on the selector cam. You’ll see what I’m talking about during reassembly.



The transfer bar slider needs to be oriented such that the long “tab” is under the transfer bar, pointing upwards.


  1. The transfer bar slider (small metal piece that sits under the transfer bar) caused some trouble for me. I reinstalled it upside down the first time I took the gun apart. I immediately noticed that my pump lock was no longer working. In addition, I would occasionally get a “dead trigger” after pumping the forend. It turns out that this little piece was responsible. The long tab needs to be under the transfer bar, pointing upwards, in order for the gun to function correctly.

  2. I omitted the shell tube storage spring/follower and shell door during reassembly. The former is useless, the latter only serves to slow down reloads.


Post-upgrade performance: all upgrade parts installed, including modified x-ring and 150% hammer spring. Testing methodology is identical to that used during phase 1.


HFC134a (with added silicon oil), 0.20g bbs, 10 shots:

Average: 271.1 fps/0.683 J

Muzzle velocity standard deviation: 5.2 fps

Energy increase from stock: 62.2%


HFC134a (with added silicon oil), 0.30g bbs, 10 shots:

Average: 246.4 fps/0.846 J

Muzzle velocity standard deviation: 11.1 fps

Energy increase from stock: 54.7%

Joule creep: approximately 23.9% more energy with 0.30g than 0.20g using duster gas


I ran out of propane, so I used King Arms green gas for the high(er) pressure gas tests. Results with propane should be similar.


King Arms green gas, 0.20g bbs, 10 shots:

Average: 450.3 fps/1.884 J

Muzzle velocity standard deviation: 6.0 fps

Energy increase from stock: 105.0%


King Arms green gas, 0.30g bbs, 10 shots:

Average: 388.5 fps/2.103 J

Muzzle velocity standard deviation: 4.8 fps

Energy increase from stock: 106.2%

Joule creep: approximately 11.7% more energy with 0.30g than 0.20g using green gas


The predicted energy increase of 96% from stock (+50% for the hammer spring, +46% for all other parts) was very close to what my results showed when using green gas. My tests with duster gas resulted in a lesser energy increase from stock.


A formal range test was not conducted because it would be entirely academic. Ballahack’s muzzle energy limit for shotguns is 1.49 J, so my 2 J shotgun was way outside field regulations. The test firing I did conduct was inconclusive due to erratic flight paths. Excess grease found its way onto the hopup rubbers and inner barrels leading to this inconsistent performance downrange.


I noticed that HFC134a gas provides 2-3 times more joule creep than propane, both with a stock gun and after installing upgrades. The expansion ratios of the two gases may be partially responsible for this phenomenon, with HFC134a at 1:228[8] and propane at 1:270.[9] I admit that the thought of a lesser expanding propellant providing more joule creep is counterintuitive, so I suspect there are multiple factors contributing to this effect.

Edited by ardrummer292
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Phase 3: Hopup system experimentation



A peculiar design: the rubber just sits on top of a cutout in the barrel.


The gun’s erratic trajectories out of the box were due to excess grease coating the hop rubbers and inner barrels. Similarly, the post-Phase 2 upgrade hopup issues were caused by more grease being fired down the barrel. I stripped the gun down and wiped off as much excess grease from the air system and barrels as I could. This involved disassembling the expansion chamber, nozzle, and hopup unit/inner barrel assembly.


Now that the source of the inconsistent performance was corrected, I could focus on trying to tweak the amount of backspin. My ideal result was enough hop to send heavy bbs (around 0.36g) on a relatively flat trajectory in 3-shot mode and moderate-weight bbs (around 0.28g) on a similar path in 6-shot mode.



A few of the force diagrams I drew up when trying to estimate the effects of various shim geometries on hopup performance. The electrical tape shim is shaded in green and the resultant force profile is shaded in pink. The numbers indicate how many squares are encompassed in the force profile, which roughly correlates to the total amount of downward pressure applied to the bb(s).


There aren’t a lot of ways to alter the amount of backspin the fixed hop provides. The following are a few of the methods I tried.


General modification and installation notes:

  • If you find the standard amount of hopup satisfactory, reinstall the hopup rubbers per factory methods. No shimming, just stock parts. This is the surest way to maintain consistent hopup performance and airseal.

  • Before re-inserting the inner barrels and hopup rubbers back into the hopup unit, I recommend applying a heavy coat of grease to the inside of the hopup unit. This will improve airseal, similar to using teflon tape on AEG hopup buckings. Since excess grease is likely to squeeze into the inside of the inner barrel and hopup rubber, have a fistful of q-tips ready to clean it out.

  • If you attempt ANY modifications, test feed each barrel before reassembling the gun. With the inner barrel/hopup unit assembly removed from the gun, drop a bb into the chamber and attempt to push it through using an unjamming rod or something similar. Excessive resistance at the hopup rubber means your gun will jam. If you need a reference for how much resistance is acceptable, pull the inner barrel/hopup assembly out of one of your zeroed guns and test feed it in a similar fashion.


If you want to increase the amount of backspin:

  • If you’d like more hopup, try shimming the top of the hopup rubbers with 1-2 layers of electrical tape. Creating a shim that covers the entire “flap” portion of the rubber will provide more hop than applying a narrower strip of electrical tape down the center of the rubber. Ensure that your electrical tape shim doesn’t sit over the circular portion of the hopup rubber, as that will lead to feeding and airseal issues.

  • If you’re having difficulty keeping your electrical tape shim in place during reassembly, try installing the hopup rubbers in the hopup chamber before installing the barrels in the chamber. While tricky and likely messy, this installation method will ensure that your shims don’t slide around or bunch up on the rubber.

  • The WiiTech hop unit is machined in such a way that it provides increased pressure on the top of the hopup bucking, almost like a permanent and more precise version of the electrical tape shim described above. The WiiTech unit has another helpful feature: there is a groove machined into the chamber end of the unit, which gives the circular portion of the hopup rubbers a place to lock in. This prevents the rubber from being pulled out of position by the cycling of the nozzle, which can lead to jamming issues. 0.28g-0.30g bbs perform best when this part is used in conjunction with stock hopup rubbers.

  • The SAT hopup rubbers are thicker and stiffer than stock TM rubbers, thereby providing more backspin when installed in the stock hopup chamber despite their smaller internal “mounds.” The amount of backspin with these rubbers installed in the stock TM hopup chamber is similar to the results you get using the stock TM rubbers in the WiiTech hopup chamber. Bear in mind that the SAT rubbers are designed differently than the TM rubbers: the muzzle-end “mound” is smaller than the chamber-end “mound,” which is the opposite of TM’s design. This means that, in 6-shot mode, the SAT rubbers impart significantly less backspin on the forward-most bb than the one behind it. The result is a large vertical spread when used in 6-shot mode, with a pattern approximately 6 feet tall at 90 feet downrange using 0.30g bbs. Compare this to the tighter pattern TM rubbers provide in 6-shot mode: 3 foot spread at 120 feet.


The following techniques may or may not work:

  • You can superglue the muzzle-end “flap” of the hopup rubber to the corresponding edge on the inner barrel.[10] This can prevent the “flap”-end of the rubber from protruding into your barrel after shimming, which can help to reduce jams. It’s easy to accidentally get superglue inside your barrel, so be careful.

  • It may be possible to teflon tape the hopup rubber to the inner barrel. This modification will improve airseal and act as a very thin tape shim, slightly increasing the amount of backspin. This is nearly impossible to do with the WiiTech fixed triple inner barrel “cluster,” but should be feasible with standalone inner barrels. Installing the inner barrel into the hopup unit will be tricky after adding teflon tape.



My shimming method for TM hopup rubbers installed in the WiiTech hopup unit. Each strip is 1/32 in wide and runs the full width of the hopup rubber. The yellow tape has no significance; I ran out of black electrical tape and yellow was all I had left.


  • When using the WiiTech hopup unit, you may be able to add a pair of narrow electrical tape shims to stock TM hopup rubbers. These shims will lay perpendicular to the path of bb travel, over the internal mounds, and can be no more than 1/32 in (0.8 mm) wide. This method gave me excellent results with 0.40g (in 3-shot) and 0.36g (in 6-shot). You must be very precise when cutting your shims; if they aren’t within a hundredth of an inch of each other, erratic patterns will result. I’m not confident that these results are repeatable, so your mileage may vary.


The following techniques are NOT recommended:

  • DO NOT install SAT rubbers in conjunction with the WiiTech hop unit. This setup yields way too much backspin. During my testing, even 0.48g bbs were massively over-hopped when using duster gas. Additionally, the pressure on the bb in the hopup unit is so great that it causes chambering issues, nozzle damage, and erratic flight paths due to the bbs getting ground against the inner barrel. A stronger nozzle shaft return spring may help with chambering, but I found no way to prevent the nozzle from being damaged (even the WiiTech aluminum nozzle got crushed). Bear in mind that the loading system struggles with heavy bbs, so even if you correct the overhop and chambering issues, the gun may not be able to consistently feed the bbs needed for optimal performance.



My final attempt at using standard shimming techniques with TM rubbers in the WiiTech hopup unit. This shim is 1/32 in wide, which is as narrow as I could reliably cut. Even this setup yielded massive overhop, sending 0.40g bbs skyward after 50 feet.


  • DO NOT use standard shimming techniques when using the stock TM hopup rubbers in the WiiTech hop unit. Even with a 1/32 in (0.8 mm) wide strip of electrical tape laid in line with the direction of bb travel, massive overhop was the result. Shims any wider than this will yield excessive hop pressure issues very similar to using SAT rubbers in a WiiTech unit.

  • DO NOT superglue tape to the top of the hopup rubbers. I had an awful time trying to get any sort of tape “shim” to stick to the hopup rubber, so I used a small dab of superglue to secure it. Bad idea. Superglue doesn’t flex very well, which led to deformation of the rubber when installed. Jams for days.

  • DO NOT superglue the perimeter of the hopup rubber to the inner barrel. I tried this in an attempt to improve airseal. I totaled a set of hopup rubbers and had a bunch of superglued debris inside my barrel. I had to discard the hop rubbers and clean the inner barrels with acetone to remove the residue.


After all this testing, I settled on using stock TM rubbers, shimmed with two 1/32 in wide strips of electrical tape laid perpendicular to the bb’s direction of travel, in conjunction with the WiiTech hopup unit. This setup provided respectable range using 0.36g bbs in 6-shot mode and impressive performance using 0.40g bbs in 3-shot mode. The bb weights are a bit heavier than I wanted to use, but the results are certainly acceptable.


Preliminary testing showed that the gun seemed to function okay, although there was something odd happening downrange. One of the three bbs consistently veered off and fell short of the target. Intrigued, I chronoed the gun to see what was happening. My results were all over the place, with the standard deviation skyrocketing to 32.9 fps (compared to about 10 fps while stock). Something was obviously wrong.


There’s only one way to troubleshoot airseal on a gun with 3 inner barrels. You have to manually muzzle-load a bb into the barrel you want to test, tap it into battery with an unjamming rod, and chrono it.[11] Then you to do it again and again and again until you have enough data for that barrel. Once you’re done, move onto the next barrel and start over. It’s a tedious process, but it provides some valuable insight into the airseal status of each barrel. Bear in mind that the data recorded from this testing is only useful when trying to compare the airseal status of barrels to each other; the velocity readings you get are not indicative of what your gun will actually shoot during normal operation.


I found that the top and shooter’s left barrel were functioning as intended, with average muzzle velocities within 10 fps of each other. The standard deviation for these two barrels was around 10 fps, which is normal. The shooter’s right barrel was a mess, with an average velocity 20 fps below the other two barrels and a standard deviation of over 30 fps. This barrel must have been responsible for the bizarre chrono readings and resulting spike in standard deviation. Now that the culprit was identified, how did I fix it?


Teflon tape was not an option, so I couldn’t fix the airseal the same way as I would with an AEG. WiiTech’s website[12] lists another way to fix airseal issues: grease. The same stuff I spent an hour cleaning out of the gun needed to be applied inside the hopup unit to fix the airseal. With some trepidation, I proceeded to coat the inside of each of the hopup unit’s 3 chambers with a fairly heavy layer of grease. I reinstalled the barrels and hopup rubbers as gingerly as I could to avoid getting grease inside them. I then removed as much excess grease as possible, using about 20 q-tips in the process.


Despite my reservations, the grease did exactly what it was meant to do. Standard deviation values dropped to around 8 fps when testing individual barrels. Chronoing all 3 barrels yielded a standard deviation of about 10 fps, close to where it was before. The airseal problem was indeed fixed.

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Phase 4: Power source conversion



The gas port: in stock form (L) and after removing the taper, drilling, and tapping with 1/4-28 thread (R).


I play year-round, so my equipment has to endure everything the east coast of the US can throw at it. That includes temperatures ranging from 20 to 105 F (-7 to 41 C). Additionally, I needed a lot of propellant readily available due to the high attendance at my home field. Much as I hated to admit it, HPA was the only way to make this shotgun viable for my purposes.


How does one go about converting the TM M870 to HPA? The most popular method I’ve seen involves running a HPA line through the fixed stock, as illustrated in eagle258’s guide:


While simple and inexpensive, this method won’t work very well when adapted to non-TM M870 stocks. Part of my project involved modifying a real stock to fit the gun, so the easy option was out the window.


I devised a different way to HPA tap the gun, one that doesn’t rely on a gas-tight seal between the stock and mechanical box. I drilled and tapped the gas port at the rear of the mechanical box, then screwed an Amped Airsoft Integrated Grip Line (IGL) directly into it. In this way, I am free to modify any stock without concern for refitting any parts of the original gas system upstream of the mechanical box.


Mechanical box gas port dimensions: all approximate dimensions are +/- 0.01 in

Stock configuration: outside of gas port is tapered, so two OD and wall thickness values are provided.

Inner diameter: 0.181 in

Outer diameter: 0.307 in

Outer diameter (at taper): 0.27 in approximate

Gas port wall thickness: 0.063 in

Gas port wall thickness (at taper): 0.045 in approximate

Depth of gas port available to thread: 0.39 in approximate


The gas port dimensions listed above were what drove my choice of IGL. All Amped IGLs are 10 inches long and use one of the following 3 thread patterns:

¼-28-2A thread: used on the Jack 1.1, F1, and F2 IGL. ¼-28-2A thread characteristics are 0.2490 in max major diameter and 0.2110 in min minor diameter.

M8x0.75 thread: used on the Fusion Engine IGL. M8x0.75 thread characteristics are 0.3150 in/8.00 mm major diameter and 0.2787 in/7.08 mm minor diameter

⅛” NPT thread: used on the M249 Fusion Engine IGL. 1/8" NPT thread characteristics are 0.403-0.405 in major diameter and 0.39 in minor diameter (approximate values, since threads are tapered).

¼-28-2A thread was the only option whose major diameter didn’t exceed the outer diameter of the gas port and minor diameter was greater than the inner diameter of the gas port. As such, I bought a Jack 1.1/F1/F2 IGL, a ¼-28 drill and tap set, and a tap wrench to modify the gas port.


If you decide to modify your shotgun in this fashion, there are a few things you should know:

  1. This is a relatively risky modification. Use a drill press, if at all possible. If not, at least use a vice to secure the gas port block. If your drill or tap go off-axis, you can irreparably damage the gas port block. The only way to fix a damaged gas port block is to replace the whole back end of the expansion chamber assembly.

  2. I’m reiterating the above point because it’s important. It’s far too easy to misalign either the bit or the tap when doing this modification by hand, unassisted. At a minimum, secure the gas port block in a vice.

  3. Disassembling the expansion chamber is advised, as is plugging the internal macro line (between the gas port block and expansion chamber) with tape or something similar. I recommend having some compressed air on hand to blow out the metal shavings when finished.

  4. Measure twice, cut once. Figure out how deep you can drill before bottoming out in the gas port. I recommend clearly marking the maximum safe depth for your drill bit and tap. This will let you know when you’re in danger of drilling completely through the gas port block, which would be bad. Wrapping tape around the bit at the appropriate distance will work, but I recommend a sharpie or paint pen since they won’t slide around on the bit. Be meticulous, mistakes can cost you.

  5. If attempting this method, you should grind off the tapered portion of the gas port (facing the stock end of the gun). After threading, the minimum wall thickness in this area would be less than 1/64 in. This is a 76% reduction in wall thickness, which will likely to lead to structural integrity issues (i.e. cracking or snapping off part of the gas port). Removing the tapered portion of the gas port still allows you to use the original stock and gas system, meaning you can keep your gun pressurized with the stock parts when not in use.

  6. Tapping the drilled gas port by hand took much more force than I expected. Make sure the handles on your tap wrench are long enough to exert a lot of force when threading the port.



Inner diameter (min) = minor thread diameter = 0.2110 in

Inner diameter (max) = major thread diameter = 0.2490 in

Outer diameter is same as above; outer diameter (at taper) is no longer applicable due to removal of taper

Gas port wall thickness (min) = [outer diameter - inner diameter (max)] / 2 = 0.029 in (54% reduction in thickness from stock configuration)

Gas port wall thickness (max) = [outer diameter - inner diameter (min)] / 2 = 0.048 in (24% reduction in thickness from stock configuration)

Depth of thread inside gas port (after removal of taper) = 0.35 in approximate

Thread length on Amped Airsoft Jack 1.1/F1/F2 IGL: 0.19 in approximate (less than 0.35 in, so the IGL can completely screw into the modified gas port)


Now that I had it hooked up to external air, I needed to figure out what output pressure was safe to use without blowing up the gun. There’s a warning that comes in the box, stating that the gun shouldn’t be used in temperatures warmer than 40 C. It’s fair to assume that the pressure of the propellant gas becomes too high at this point, which causes damage. How can I apply that information to HPA?


Simple, really. Figure out the gauge pressure of HFC134a (the only gas approved by TM) at 40 C and you’ve got your upper limit for pressure. This came out to be about 133 psi gauge (psig).[13] As such, I determined my maximum allowable pressure at the regulator would be 120 psi in order to maintain a small (9.8%) factor of safety.


I followed Ballahack chrono SOP to adjust my output pressure. Per their procedure, all gas/HPA weapons and sniper rifles are chronoed with 0.40g bbs. I tweaked the regulator until I achieved my desired muzzle velocity. The results are as follows:


For reference - HPA @ 72 psi, 0.20g bbs in 3-shot mode, 20 shots:

Field limit: 400 fps/1.49 J

Average: 363.6 fps/1.228 J

Muzzle velocity standard deviation: 9.8 fps


HPA @ 72 psi, 0.40g bbs in 3-shot mode, 20 shots:

Field limit: 283 fps/1.49 J

Average: 267.2 fps/1.326 J

Muzzle velocity standard deviation: 10.4 fps

Joule creep: approximately 8.0% more energy with 0.40g than 0.20g using HPA @ 72 psi


HPA @ 72 psi, 0.36g bbs in 3-shot mode, 15 shots:

Field limit: 298 fps/1.49 J

Average: 284.1 fps/1.350 J

Muzzle velocity standard deviation: 11.5 fps

Joule creep: approximately 9.9% more energy with 0.36g than 0.20g using HPA @ 72 psi


Ammo selection and range: using WiiTech hopup unit, TM hopup rubbers shimmed in accordance with my notes in Phase 3, WiiTech extended inner barrels, and all upgrade parts listed in Phase 2.

3-shot mode, HPA @ 72 psi: 0.40g bbs work best, approximate range 200 ft

6-shot mode, HPA @ 72 psi: 0.36g bbs work best, approximate range 180 ft


I was also interested in the number of shots I could get out of my HPA tank setups. I found an equation[14] that should provide a ballpark figure:


N = (V * P_t) / P_r


N = number of shots

V = volume of tank (or tanks, if more than one), in cubic inches (ci)

P_t = pressure inside tank(s), in psi

P_r = output pressure of regulator, in psi


Plugging in the values for my First Strike Hero 2 48 ci 4500 psi tank yields:


N = (V * P_t) / P_r = [ (48 ci) * (4500 psi) ] / (72 psi) = approximately 3000 shots in 3-shot mode per 48/4500 tank, equivalent to 300 shells


Similarly, substituting the values for my dual tank rig using two First Strike Guerilla 13 ci 3000 psi tanks gives me:


N = (V * P_t) / P_r = [ (13*2 ci) * (3000 psi) ] / (72 psi) = approximately 1083 shots in 3-shot mode with dual 13/3000 tanks, equivalent to 108 shells


This increased shot capacity is a massive improvement over the 100 shots/10 shells[15] available from the stock gas tank. A bit overkill, absolutely, but I’d rather have too much propellant on hand than too little.

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Phase 5: Repairs, reinforcement, and loading system upgrades



WiiTech provides upgraded nozzle shuttle springs with their top gas nozzle.


The WiiTech top gas nozzles I bought came with stronger nozzle shuttle springs for use with higher pressure gases. I believe these nozzle shuttles automatically throttle propellant flow to each inner barrel, leading to more consistent velocities and patterning. The operating pressure I determined for the gun, 72 psi, is more readily associated with duster gas than propane: at 75 F/23.9 C, HFC134a puts out about 79 psi while propane puts out around 120 psi. As such, I decided to reinstall the (presumably weaker) stock TM nozzle shuttle springs in order to ensure proper nozzle shuttle function.



One of the WiiTech nozzles damaged during the course of testing. This one was partially crushed from hopup experimentation, then the center pin bent during a jam.


I managed to severely damage two separate WiiTech nozzles during the course of my testing. One nozzle was partially crushed by repeatedly chambering bbs under excessive pressure from the hopup, which occurred during Phase 3 testing. This same nozzle bent its center pin during an especially nasty jam. The loading gate (P/N 870T-20) was inadvertently superglued open, which resulted in bbs getting chopped and jamming the feed system, causing the ensuing damage. The second nozzle snapped its center pin during a jam caused by the same type of glue-induced loading gate malfunction. I replaced these damaged nozzles with a WiiTech HFC134a nozzle, which appears to be identical to the top gas model less the inclusion of the upgraded shuttle springs, nozzle shaft return spring, and o-ring.


I decided to reinforce some parts while I had the gun open. The pump lock secures the forend in the forward position after chambering a round. It’s notoriously easy to break when slam-firing, which I did quite a bit during testing. I bought the WiiTech hardened steel lever set (commonly known as the “pump lock,” P/N 4052) and installed it. The gun will function fine without it, but I’d like to be able to verify that I have a round chambered on patrol with a quick tug on the forend. No one likes to line up a perfect shot just to have a dead trigger or double feed ruin the moment.


I also added a DangerWerx transfer bar to my order as a preemptive upgrade. I opted for the aluminum DangerWerx part over the steel SAT option due to fitment concerns.[16]



Top: Stock loading plate spring.

Bottom: Pre-loaded Blackcat spring.


I noticed an alarming amount of misfeeds when using 0.40g bbs. The gun would frequently chamber only 2 bbs when in 3 shot mode and 4 bbs when in 6 shot mode. From my preliminary research, I knew that the stock loading system struggles with bbs heavier than 0.28g. I decided to upgrade the loading plates and corresponding springs in order to feed the 0.40g bbs used by my home field’s chrono station. I purchased the Blackcat mechanical box spring set as well as WiiTech aluminum loading plates (P/N 4044). The former provides stronger replacement springs for everything in the mechanical box, including the loading plates. The latter is a stronger, lightweight version of the stock part, and is reported to work well with heavy ammo and smooth out misfeed issues. I bent the loading plate springs in accordance with a guide online; this is done in order to pre-load them, increasing the spring force they exert during the feeding process.[17] I chose the aluminum WiiTech loading plates instead of the steel SAT equivalent due to part interaction concerns[18]: aluminum is less likely to grind down the tracks on the inside face of the pump arms, which would surely lead to feeding issues that could only be solved by expensive and hard-to-find replacement pump arms.



Top: stock TM follower assembly mounted on stock TM feed spring.

Bottom: HexMag follower base and stock TM follower top mounted on MAS SuperFeed spring.


Of course, the loading plates and springs are only one part of the loading system. The other, equally important part are the shotshells themselves. Not surprisingly, TM (or any other manufacturer) doesn’t design their shotshells to reliably feed 0.36+g bbs at the 53 rounds per second required for the gun to function correctly.[17] In order to rectify this issue, I purchased Maple Airsoft Supplies SuperFeed springs and HexMag HexID magazine followers. I used cut-down sections of these springs in conjunction with HexMag follower bases to modify my shotshells. The TM follower base is small, so it doesn’t line up correctly with the wider SuperFeed spring coils. I’m using just the base of the HexMag follower because the whole HexMag follower assembly is too long to feed in the shell. Through some measurements, math, and a bit of experimentation[19], I determined the optimum feed spring length to be 42 coils. This is about 9.5 inches for an uncompressed spring, fresh out of the package. The round count of each shell is reduced to 25 bbs, which is an unfortunate side effect of installing a more heavy duty spring.



The left-side pump mechanism mentioned below.


Speaking of springs, I replaced some of the other mechanical box springs with those provided in the Blackcat spring set. I installed the following:

Pump lock springs: Upgrade in conjunction with the pump lock mechanism itself.

3/6-shot selector cam spring: There are (admittedly dated) reports of the stock selector cam spring being too weak to ensure proper feeding, especially after the gun is subjected to heavy use. While I had not experienced any issues related to such an issue, I decided to install this part as a preventive measure.

Shell release lever spring: I removed the shell door, so stiffening this spring is solely for my peace of mind.

Left-side pump mechanism spring: This is the gray metal spring-loaded slider inside the mechanical box that interacts with the left pump arm. I’m not entirely sure what function this mechanism performs, but I decided to install the upgraded spring for it anyway.


I did not install all the springs included in the Blackcat mechanical box set. The following were omitted or found unsuitable:

Nozzle shaft return spring: WiiTech includes an upgraded nozzle shaft return spring with their top gas nozzle, so I had to choose between Blackcat and WiiTech components. Due to my faith in WiiTech’s products (and the fact that the WiiTech spring hadn’t caused any issues), I stuck with it.



A standard AEG gear shim installed under the 3/6-shot selector slider screw.


I disassembled the gun over 20 times while installing upgrades and experimenting with the hopup system. Throughout the course of this testing, I stripped out the threaded holes in the loading gate (plastic, P/N 870T-21), the threaded hole for the 3/6-shot selector slider (plastic), and the bolts for the outer barrel saddle (P/N 870T-23). I bought new M3x0.5 25mm long bolts, ground them to size, and installed them in the outer barrel saddle. This fixed the barrel wobble induced by lack of tension in the outer barrel assembly. I re-threaded the stripped holes in the plastic components with a carefully applied single droplet of superglue in each. I also added a standard AEG gear shim under the screw used to secure the 3/6-shot selector slider in order to increase the pressure applied to that part, which should reduce the likelihood of it slipping out of place.

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Phase 6: Aesthetic modifications


Now that I had the gun repaired and performing well, it was time to dress it up.



“I like to keep this for close encounters.” UKSF in Afghanistan, circa 2012.


I wanted to kit the gun out in line with the picture above. The external parts list I assembled is as follows:

Knoxx SpecOps retractable stock, Gen 1: These have been discontinued for a few years and can be difficult to find.

Blackcat sling adapter: For mounting a single point sling between the receiver and stock. The right side sling loop has been ground off.

G&P mid length rail with shell carrier, P/N MSP-017: Even after cutting this rail to length, it’s not exactly the same as the one pictured above. Unfortunately, the only replica of the correct rail (available from WiiTech) doesn’t come with a side saddle, which is essential for fast reloads.

Real GG&G QD T1 mount: No replicas available, lens covers removed as pictured.

Dytac T1 with WiiTech lens protector: The real GG&G T1 mount doesn’t play nice with G&P, Element/Aim-O, and most other replica T1s, so I had to find a 1:1 scale replica. The new gen G&G GT1 has the brightness dial on the wrong (left) side and the Nuprol is hard to find stateside, so Dytac was my choice.

Tokyo Marui M870 Tactical black faux bolt, P/N 870T-39: The stock chrome bolt cover didn’t take paint very well (even after sanding and priming), so I purchased a new one in black from Eagle6.

G&P railed forend, P/N MSP-002H, with modified Longshot Mfg. M-Lok 25-slot side rails: This particular setup is as close as I could get to the proper railed forend, which is the Surefire M69. I didn’t want to irreversibly alter a hard-to-find and expensive real rail system, so I settled on something that wouldn’t alter the gun’s functionality and looked fairly close.

Replica H&K vertical foregrip: As pictured, listed on eBay as “Aluminum Core Round Handle Regular Vertical Grip” for 11 USD.

FMA LA-5 battery box: As pictured, placeholder for regular weekend play when I don’t need IR aiming capabilities.

Night Evolution M600C weapon light: A plausible choice and necessary addition.

WiiTech dummy magazine extension, P/N 4028, mounted with coupler from G&P dummy magazine extension: WiiTech is the only company that makes a realistic replica of the factory Remington +2 magazine extension, while G&P make the correct ribbed coupler (vice the knurled coupler included with the WiiTech).

Real Remington magazine tube clamp: As pictured.



Real Remington 870 stock, before (L) and after (R) modification.


The stock mounting post at the back of the TM’s receiver wasn’t concentric with the mounting hole in the Knoxx stock. In addition, the diameters of the mounting interfaces were vastly different: 0.355 in outer diameter for the Marui’s post, 0.621 in inner diameter for the Knoxx’s hole. While I probably could have bodged the stock on with minimal modification, the empty space resulting from the disparity in diameters would have lead to uneven stress distribution and eventual breakage of the stock mounting post at the rear of the receiver. I wanted the modification to be executed with precision that my hand tools couldn’t deliver, so here’s what I did:

  1. The stock mounting hole had to be filled in and new holes had to be drilled for the mounting post and gas port.

  2. I packaged the stock and Blackcat sling adapter together and passed them off to a machinist.

  3. The machinist tapped the existing mounting hole in the Knoxx stock, then used a threaded aluminum rod to fill it in.

  4. He used the sling adapter as a template in order to locate the new holes for the stock bolt and gas port in the Knoxx stock.

  5. The hole for the stock bolt was drilled: 0.375 in diameter, counterbored to 1 in depth. The counterbore houses the 0.48 in long stock mounting post. An additional 0.219 in (7/32”) concentric thru hole was made for the stock bolt.

  6. The hole for the gas port is a simple 0.375 in diameter thru hole, which will allow the IGL to pass through the stock assembly.

  7. The recess for the trigger guard had to be milled out. The excess “shelf” of material is visible in the picture on the left above, marked with green paint pen.

  8. After drilling and grinding, the machinist proceeded to grind down the stock’s mounting surface in order to reduce the gap between the back of the trigger guard and the grip.

  9. Once this was done, a new lip was machined around the perimeter of the mounting surface. This was made to fit the Blackcat sling adapter.

  10. After grinding and fitting was complete, the stock was returned to me. I purchased a M5x0.8 thread, 70mm long bolt to secure the stock.

  11. I removed the receiver shell from the rest of the gun, then bolted on the sling adapter and stock. I marked the excess length on the stock bolt and cut it to the correct length.

  12. While mounted, I noticed a small gap (less than 0.1 in) remaining between the trigger guard and grip. I applied a layer of RTV sealant to fill this gap.

  13. After cutting the stock bolt down, I reassembled everything. Modification complete.



Surefire M69 railed forend (L) compared to my modified forend (center) and standard G&P railed forend (R). My forend isn’t exactly correct, but it is closer than the other off-the-shelf options.


Finding a railed forend was surprisingly difficult. I did not intend to chop up a rare 200+ USD Surefire rail; the risk of destroying the part was too great, even with a machinist’s help. I value proper weapon function over aesthetics, so I restricted my choices to rail systems made for airsoft replicas. The G&P fully railed forend (P/N MSP-002F) has been out of production for ages and the visually identical WiiTech railed forend (P/N 4029) was 4 months away from the next production run, so I decided to try something on my own. I bought one of the readily-available G&P half-railed forends (P/N MSP-002H) and found some side rails that more closely resembled the “scalloped” type used on the Surefire rail. I cut these Longshot Manufacturing 25-slot M-Lok rails to the correct length and ground down the front of the rail system a bit in order to keep the front of the assembly flush. I then passed the M-Lok rails off to a machinist, who drilled a series of lightening holes and ground down the back face to reduce their overall height. After mounting the modified rails with some additional M5x0.8 bolts, I had created a passable stand-in for the genuine item.



Magazine extension comparison:

(L): Real Remington 870 with 18.5 in barrel and factory +2 magazine extension. Note that the muzzle is flush with the end of the extension.

(C): Real Remington 870 with 18 in barrel and factory +2 magazine extension. Note that the muzzle is approximately even with the step in the extension.

(R): TM M870 mocked up with real Remington +2 magazine extension. Mild disappointment.


I found a factory Remington mag tube clamp and +2 extension on eBay for a steal. Even though I only needed the clamp, I was curious to see how far off scale TM made their shotgun. Based on my measurements, the Marui outer barrel comes out to around 17.45 in long. This is noticeably shorter than commonplace 18 or 18.5 in bbl 870s. I was wondering why the gun looked a bit off.


It is possible to buy a die and modify a real magazine extension to fit, if you so desire. The thread on the TM is equivalent to M25.5x1.0, which can be cut with a M26x1.0 die and some finagling. The outer diameter of the real-steel Remington extension is 25.38 mm, so it is feasible to cut it down and thread it for use on the TM replica.



Muzzle-end internal threads sanded down on a G&P magazine extension coupler.


If you want to use a magazine tube-mounted forward sling plate and would like the end of your dummy extension to remain flush with your muzzle, there’s an easy way to make that happen. Grind down the threads inside the muzzle end of the magazine extension coupler, which will allow the dummy magazine extension to thread further into the coupler and keep the front of your gun nice and flush.


Magazine extension clamp placement is a minor detail, but one of the few that costs nothing to do correctly. Since the WiiTech dummy mag extension isn’t the correct length, I decided to place the clamp in the correct location proportional to its shorter overall length.

Real Remington +2 extension overall length: 6.46 in

Real Remington +2 extension location of indented ring (for mounting clamp bolt): 2.52 in back from muzzle end

Percent of magazine extension forward of indented ring: 39.0%

WiiTech dummy magazine extension overall length: 5.31 in

Proportionally correct location for clamp placement on WiiTech dummy magazine extension: 0.390 * (5.31 in) = center of clamp 2.071 in (approximately 2 1/16”) back from muzzle end


My sling is the most visible departure from the reference picture above. It’s a US-issue padded 2 point machine gun sling run as a convertible 1-to-3 point by way of an ITW MQRB rigged in. The male portion of the MQRB is threaded on the sling while the female portion is bolted to the rail system. With this setup, I can instantly convert the sling from a 3-point (for patrolling or going hands-on) to a 1-point (for shooting) and back. I find it useful, but it’s not everyone’s cup of tea.


After adding a couple modified KAC rail covers (for cable management) and a quick paintjob, I had my finished product:




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Phase 7: Support equipment




At this point, I still didn’t have what I needed to use it on the field. Here’s what I was missing.


Load carrying equipment: When your firepower is limited, speed equals survival. I set up two lightweight rigs for this gun: a Blackhawk helivest for door-kicking and a UK-issue Northern Ireland chest rig for recce.


Air rig: I had never owned an HPA weapon before, so I built my setup from scratch. I opted for a First Strike Hero 2 48/4500 tank for my NI chest rig due to its small size, coming in at only 7.75 in tall with a 4.5 in diameter, which is barely small enough to fit in the chest rig’s utility pouch. For the helivest, I used a pair of First Strike 13/3000 tanks hooked up with a dual tank rig. The 13/3000 tanks are a good fit for the hydration pouch on the helivest, at 8.75 in tall and 2 in diameter. Both setups use a Redline SFR regulator, which are great since they have integrated tournament locks and don’t require the use of SLP tanks. I used 1/8 NPT 90 degree adapters on both regulators in order to reduce the profile of the air line going into the regulator. I rigged the air line into the right side shoulder strap of both rigs to minimize snagging when moving through heavy brush.


Hydration: Both of my rigs feature a 1L hydration bladder. The helivest has its water source co-located with the air tanks in a hydration pouch, while the chestrig has it stowed in the utility pouch on the left side.


Magazines (or shells, in this case): I understand the appeal of loading up on cheap-o ACM shells, much like I understand why people prefer to buy cheap AEG magazines. After many years of dealing with misfeeds at unfortunate times, I decided that I’d pay pretty much anything for a well-engineered magazine that works every single time without fail. Because of this, I’ve exclusively been using Marui 68 round lowcaps for my M4-pattern rifles for years. Similarly, I decided to purchase TM shotshells: 22 in white and 8 in green, with the different colors of shells indicating the bb weight loaded within (for either 3-shot or 6-shot mode). While the shells did require a bit of additional effort to feed the requisite heavy weight bbs (see Phase 5 notes), they now perform flawlessly.


Sidearm: With the low ammo capacity of a shotgun, having another weapon handy is a smart choice. I carry either my TM P226E2 or my Cyma G17 AEP (converted semi auto-only CM.030), depending on the weather. Both are equipped with a replica Surefire X300 and their own modified Safariland 6004.


Grenades: In bad-breath-distance gunfights, being able to eliminate someone without simultaneously offering them a shot at you wins individual engagements. Besides, I don’t have the ammo capacity to pre-fire corners and boost my chances of success. My home field doesn’t allow pyro, so I use a combination of APS/Hakkotsu Thunder B’s and AI Cyclone grenades.

Comms kit: Teamwork and coordination are vital, especially when operating with less firepower on tap than a kid with a rental. Every rig I have is hooked up with accessories to support my radio of choice, a Motorola HT1000.

Edited by ardrummer292
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On the field



My shotgun, in use. It wasn’t externally complete at this point, hence the lack of spray paint.


While it may have started out as a mild-mannered replica, this thing is now a powerhouse. The hopup modifications I performed allow me to run fairly heavy bbs, meaning light brush is no longer cover. I run the gun almost exclusively in 6-shot mode, with the resulting spread allowing me to land hits on the smallest exposed bit protruding out from barricades and doorways. I hold 3-shot mode (and the accompanying heavier ammo) in reserve for longer-range standoff engagements, which the gun can handle but I try to avoid all the same.


This platform isn’t without drawbacks, which neatly balance out the benefits. Ammo management is a constant battle. Much like with a real shotgun, if you aren’t shooting, you better be loading. I know there are adapters and drum mags to negate this issue, but I feel that such accessories cheat you out of an essential part of the shotgun experience. Another key point is mobility: getting pinned down without support will likely lead to your demise. The shotgun’s fire rate likely isn’t enough to effectively counter-suppress and withdraw, so lean on your rifle-wielding buddies when things get heavy. Exploit your lighter load and try to close with the contact from a different direction.



Some helmet cam footage of a “chaos” game at my local field. This game allows dead players to be absorbed by another team, so group cohesion took priority over aggressive individual movement.


Verdict: Lessons learned, and was it worth it?


Experimenting with the hopup system was far and away the most tedious and time-consuming part of this project. It took me no less than 60 hours of repeated disassembly, shimming, reassembly, and testing to figure out my shimming method. That said, I’m glad I did it. Thanks to my findings, this gun neatly fills the capability gap I intended to address.


This shotgun is not designed to be disassembled 20+ times. Threaded holes in metal and plastic alike will strip with extensive use. My advice to you? If you’re going to perform upgrades or modifications, do them all at once to minimize the amount of times you have to take it apart.


TM’s shotgun shells caused me a bit of trouble. Unlike their other magazines, these shells required some modifications and a break-in period before feeding heavy bbs consistently. Some of the issues I experienced were due to the system getting pushed past its intended performance envelope, so I’m not too disappointed.


This gun does not like soft or brittle bbs. The jams I experienced (which resulted in extensive nozzle damage, as detailed in my Phase 5 notes) were mostly due to the loading gate being stuck open, but the ammo I was using is partially responsible. Valken bbs are notoriously soft and easy to chop, which undoubtedly contributed to the severity of the jams and ensuing damage. Once I switched to BLS/HPA bbs (my preferred brand), I had no further jamming issues.


If I could do it all again, I would have never considered any SAT-made parts. They are not as well engineered as WiiTech, both in material selection and design. The SAT hopup rubbers are the one possible exception to this, although they are not without their own drawbacks (see: reduced 6-shot mode performance).


All technical information aside, there is one question that remains unanswered. The most important question of all: was it worth all the research, experimentation, effort, and money?






What’s next?


I plan on getting in touch with LC’s Engineering Outpost[20] to see if I can get some steel forend arms (TM P/Ns 870T-10 and 870T-11) made, in the interest of longevity. TAPP Airsoft have also hinted at the possibility of releasing CNC’d replacement arms for full-size M870s.



British SO19 operator with 20 in bbl L74A1 (M870). I will have to replicate the 18.5 in bbl variant since no one makes longer outer barrels for the TM M870.


I’m also in the process of building a second shotgun, with the desired end result approximating the British L74A1. An old-school top folding setup will go nicely with older kits, especially my recently-completed early 2000s SAS black kit. I’m hoping the lessons I learned from my first build will make this new gun cheaper and easier to put together.

Edited by ardrummer292
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Additional Resources


Arnie’s Airsoft review of the TM M870 (some of the information within is quite dated, so keep that in mind): https://arniesairsoft.co.uk/forums/index.php?/topic/210618-tokyo-marui-m870-tactical-gas-tri-shot-shotgun-picture-text-and-video-review/

Arnie’s Airsoft TM M870 fix and tuning thread:

The Real and Dirty on the Tokyo Marui M870 Gas Shotgun, by 258eagle et al: https://www.airsoftsociety.com/threads/the-real-and-dirty-on-the-tokyo-marui-m870-gas-shotgun-hpa-tm-m870.120317/

Gun-Zuki Web Studio blog post with detailed disassembly pictures (text in Japanese): http://blog.livedoor.jp/calsonicskyline12/archives/32989442.html

Gungineer review with detailed disassembly pictures (text in Japanese):


TM M870 video playlist covering disassembly, troubleshooting, and reassembly: https://www.youtube.com/playlist?list=PLNdyNaSlmu2PFP29Kxuw-SD3nEWkp_mBE

TM M870 and KSG Owner’s Group: https://www.facebook.com/groups/1503954529842211/

WiiTech M870 parts: http://www.wiitech.com.hk/product-list.shtml?path=20_88

OEM TM M870 parts from Eagle 6: https://eagle6.co.uk/shop/genuine-oem-parts/parts-tokyo-marui-gas-shotgun-series/parts-tokyo-marui-m870-gas-shotgun-series.html

Amped Airsoft Integrated Grip Line: https://ampedairsoft.com/amped-integral-grip-line-standard-weave-igl-hpa-grip-line/

First Strike Hero 2 48/4500 HPA tank: https://www.ansgear.com/First_Strike_Hero_2_Carbon_Fiber_Air_Tank_48_450_p/fshero2cfairtank48black.htm

First Strike Guerilla 13/3000 HPA tank: https://www.ansgear.com/First_Strike_Aluminum_Air_Tank_13_3000_p/fsairtank13.htm

Airsoft Junkiez Dual Tank Redline SFR Rig: http://www.airsoftjunkiez.com/airsoftjunkiez-dual-tank-sfr-regulator-13ci-26ci/

TM M870 manual: https://www.tokyo-marui.co.jp/pdf/p_pdfmanual_140521161324.pdf




[1]: https://www.airsoftforum.com/topic/28824-firepower-is-nothing/

[2]: http://www.thefixer.biz/2013/03/03/shotgun-muzzle-energy/

[3]: https://www.kramerdefense.com/v/vspfiles/assets/images/5.56_NATO_M4-14.5_bbl_&_7.62_x_39_AK-16.0_bbl.pdf

[4]: http://donaldmsensing.blogspot.com/2003/06/infantry-rifle-combat-distances.html

[5]: http://www.brassfetcher.com/Shotguns/Maximum%20Effective%20Range/Buckshot.html

[6]: https://youtu.be/jccG8NoHmSk

[7]: https://www.youtube.com/watch?v=bQvXaIS6g_E&list=PLNdyNaSlmu2PFP29Kxuw-SD3nEWkp_mBE&index=8

[8]: https://hvac-talk.com/vbb/showthread.php?1539911-R134a-Sample-Recovery-Disposal&styleid=41

[9]: http://www.grillnpropane.com/html/properties.html

[10]: http://translate.google.com/translate?u=http%3A%2F%2Fgungineer.matrix.jp%2Fairgun%2Ffix%2FTM%2FM870%2FM870_tantical.htm&langpair=ja%7Cen&hl=en&ie=UTF-8&oe=UTF-8

[11]: https://youtu.be/gZgLgd6iZkQ?t=1595

[12]: http://www.wiitech.com.hk/product-detail.shtml?product_id=676&path=20_88

[13]: https://www.chemours.com/Refrigerants/en_US/assets/downloads/freon-134a-thermodynamic-properties-eng-units.pdf

[14]: https://www.youtube.com/watch?v=06dBmrB8a7E

[15]: https://arniesairsoft.co.uk/forums/index.php?/topic/210618-tokyo-marui-m870-tactical-gas-tri-shot-shotgun-picture-text-and-video-review/&do=findComment&comment=2640935

[16]: https://youtu.be/gZgLgd6iZkQ?t=1124

[17]: https://www.neasg.org/armory-upgrade-and-repairs/22564-tokyo-marui-m870-fixes-tuning-thread.html

[18]: https://youtu.be/gZgLgd6iZkQ?t=1388

[19]: https://docs.google.com/spreadsheets/d/1vpi1Ja1Etj-_a33EwOU9B_4w1-NiFyPq_5vcFCTgK2E/edit?usp=sharing

[20]: https://www.facebook.com/lcsengineeringoutpost/

Edited by ardrummer292
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Addendum #1: Shotgun number 2 complete!




TM-based L74A1 project, completed.


This second shotgun was significantly easier to complete, largely thanks to the experience gained from my initial build. The following is the parts list for this gun:

  • Base gun: Tokyo Marui M870, wood version

  • Internals (almost identical to my first build)

    • Amped Airsoft Integrated Grip Line (IGL) for Jack 1.1/F1/F2, screwed into ¼-28 tapped gas port

    • Blackcat mechanical box spring set

    • WiiTech 150% hammer spring, P/N 4051

    • WiiTech stainless steel valve pin, P/N 4043

    • Size 104 nitrile x-ring, modified, installed on valve pin

    • DangerWerx transfer bar

    • WiiTech aluminum loading plates, P/N 4044

    • WiiTech duster gas nozzle, P/N 4045, with nozzle shaft return spring from WiiTech top gas loading nozzle

    • WiiTech 2016 version hopup chamber, P/N 4059

    • TM hopup rubbers, shimmed with electrical tape

    • WiiTech 6.03 x 380mm extended inner barrel set, P/N 4032

    • Blackcat trigger spring

  • Externals

    • G&P top-folding M870 stock, P/N GP385, machined to fit

    • Homemade rear sling point: Stryker armored vehicle seat belt anchor (NSN 2590-01-551-0614) bolted to stock hinge via ½-20 UNF x ¾ bolt (stock hinge had to be tapped)

    • TM M870 Tactical black faux bolt, P/N 870T-39

    • TacStar 870 6 round side saddle, modified to fit TM shotshells

    • Real-steel Remington 870 rear sight ramp assembly and front sight ramp assembly, JB Welded to outer barrel

    • G&P replica Surefire pump, P/N MSP-005, bezel ring ground down flat to more closely resemble older Surefire lights

    • Blackcat front sling mount

    • WiiTech dummy magazine tube extension, P/N 4028, with coupler from G&P magazine extension. Mag tube coupler muzzle-end threads have been ground down in order to keep end flush with muzzle.

    • Real-steel Remington 870 magazine tube clamp, blued finish

    • Homemade quick-adjust 2 point sling, made with 1 ¼ webbing and Magpul MS1 sling hardware

  • Accessories/other (shared with my first build)

    • TM shotgun shells with MAS SuperFeed springs and HexMag follower bases

    • Redline SFR regulator

    • First Strike Hero 2 48/4500 HPA tank, or dual First Strike Guerrilla 13/3000 HPA tanks

    • Amped Airsoft HPA line

    • TechT GunSav grease


The following are some additional lessons learned from this endeavor:


If you choose to perform the x-ring modification, be careful when grinding the x-ring down. If you don’t keep a close eye on your file/sandpaper, you are likely to start accidentally grinding the valve pin as well as the x-ring. This creates metal shavings, which embed in the rubber and damage it. Also, be careful about how fast you grind the x-ring down. If you go too fast, the rubber will begin to lose its integrity and will compromise durability. If you see striations on the grinding surface of the rubber, you’re going too fast. Lastly, be sure to grind the x-ring into the correct size. Too small and it won’t seal properly, which will cause leaks. Too big and it will cause the valve stem to stick, resulting in very low muzzle velocity readings.


Keep an eye on your trigger pin (P/N 870T-45) when removing or installing your trigger group. Mine slipped out of the trigger group during reinstallation, which led to the trigger spring (P/N 870T-44) launching itself into the inside of the mechanical box. While function-checking my gun, I noticed some peculiar jamming issues. Nothing seemed out of order until I noticed that the trigger itself had no spring tension on it. I removed the trigger group, fished out the (now mangled) trigger spring, and installed a Blackcat replacement. Thankfully, no additional damage was caused by the wayward spring wedging its way into the working bits of the gun. I added a small dab of Loctite to each end of the trigger pin in order to prevent this same issue from happening again.



G&P top-folding M870 stock:

(L): Out of the box.

(C): Mounting surface after modification.

(R): Safety button alignment and stock arm modification. Also pictured is my home-brewed rear sling mount.


Not surprisingly, the G&P top folding stock (made for their single-shot spring M870 series) is not a drop-in fit for the TM shotgun. I contacted my machinist buddy, the same one who modified my Knoxx stock. He was kind enough to do a similar modification to this stock. It turns out that getting this one to work was a bit more tricky.

  • Mounting surface modification:

  1. The stock mounting hole had to be filled in and new holes had to be drilled for the mounting post and gas port. A similar situation to the one I dealt with before.

  2. The machinist tapped the existing mounting hole in the folding stock, then used a threaded aluminum rod to fill it in. He ended up removing a thin strip of plastic on the bottom side of the stock mounting hole, since it was too thin to take any load.

  3. Instead of using the Blackcat sling adapter as the stock-to-receiver interface (as was done with my first build), he chose a more elegant solution. Through some fancy milling work, he created the correct profile on the stock’s mounting surface itself so it would snugly lock into the receiver. He opted for this more labor-intensive method in order to ensure that the entire perimeter of the stock mounting surface would engage with the back of the receiver, resulting in a stronger assembly. I can’t even pretend to explain the specifics of how he did it, so hopefully the picture helps.

  4. While filling, drilling and shaping, he was mindful of the location of the safety button. The mounting surface was ground down until the safety button was perfectly aligned in the provided circular recess on the pistol grip.

  • Mounting hole and gas port hole drilling:

  1. The hole for the stock bolt was drilled: 0.375 in diameter, counterbored to 1 in depth. The counterbore houses the 0.48 in long stock mounting post. An additional 0.219 in (7/32”) concentric thru hole was made for the stock bolt. This much is identical to the Knoxx stock.

  2. The hole for the gas port is a simple 0.375 in diameter thru hole, which will allow the IGL to pass through the stock assembly. Identical to my first modified stock, with one added complication.

  3. The gas port hole happened to run straight through where the stock folding release button was located. He removed the spring-loaded button and fabricated a short dowel of the same diameter. He then pressed it into the grip, effectively fixing the stock in place. This folding stock no longer folds.

  4. After all this was complete, he took the M5x0.8mm bolt I supplied, turned down the diameter of the bolt head a bit, and cut it to length. The diameter of the bolt head was reduced in order to mitigate any sharp edges produced near the backstrap of the grip.

  • Stock arm modification:

  1. He re-shaped the forward end of the left stock arm in order to eliminate interference between it and the side saddle shell carrier. After removing this material, the stock would no longer lock in the folded-up (or “closed”) position. This was not a concern for me, since I value a side saddle more than the folding feature of this stock.

  2. He removed 2 ½ inches from both stock arms and remade all the needed geometry for the buttpad to function. This had the effect of shortening the length of pull significantly, keeping it even with my first build as well as all my rifles. While a bit awkward to shoulder when slick, the short stock comes into its own when wearing body armor.

  • Modification complete. And no, he doesn’t want me to share his contact information. Sorry.



The method I used to modify the TacStar side saddle to fit TM shotshells.


I used a real-steel TacStar side saddle for this build. Since TM-spec shotgun shells are significantly wider than their real 12 gauge counterpart, some modifications were required in order to get it to work.

  • I dismounted the plastic shell holder from the metal mounting plate by removing the 6 allen screws holding it in place.

  • I wrapped a large drill bit with duct tape until its diameter matched the diameter of a TM shotshell. Calipers help when doing this, but they aren’t necessary.

  • I then secured a strip of sandpaper to the tape-covered bit. I started with 50 grit to remove the bulk of the material, then switched to 200 grit to smooth it out.

  • I stuffed the sandpaper-covered bit into each shell slot and drilled for about 60 seconds, constantly moving the bit back and forth to ensure even removal of material. The strip of sandpaper was replaced after grinding out every couple of shell slots.

  • Once I was done grinding the shell slots to size, I thoroughly rinsed the shell holder off. This was in order to remove the rather significant buildup of plastic dust.

  • I re-mounted the shell holder on the metal mounting plate, then loaded it with shells and let it sit for a few days. This is so the shell slots could re-form to their new size.

  • Modification complete!

Thankfully, the trigger group “pins” (actually bolts) included with the TacStar side saddle are the exact right diameter and length for the TM M870. No additional modifications were necessary.

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Addendum #2: Wear and breakages



The mounting protrusion for the rearmost pump lock mount broke off. The point of breakage is circled in red. The factory-designed gap between the mounting protrusion and lower edge of the mechanical box is roughly outlined in blue. This gap allows the mounting protrusion to flex up and down, which may be necessary to the function of the gun.


While performing a routine re-zeroing of the T1 red dot mounted on my first shotgun build, I began to experience peculiar Failure To Fire (FTF) incidents. When the trigger was pulled, the gun would emit a click similar to pulling the trigger on a real shotgun with an empty chamber. Upon disassembly, I noticed that the rearmost pump lock component was wiggling left to right. After removing the transfer bar to get a better look, the rear pump lock component fell off along with its plastic mounting protrusion and screw. I attempted to superglue this plastic protrusion back onto the mechanical box shell, to no avail.



Top: DangerWerx transfer bar, used and damaged.

Bottom: Stock TM transfer bar, unused.


I took a look at the transfer bar after removing it during the troubleshooting mentioned above. The DangerWerx transfer bar had become severely bent, likely due to interference with the pump locking mechanism. After comparing the location of the damage on the aftermarket component to the design of the TM part, I decided to revert to the stock transfer bar. I’m fairly certain that the stock part is designed better and is less likely to fail.


Now that the troubleshooting was done, I had to weigh my options. I could either remove the pump locks and hope that missing a chunk of my mechanical box wouldn’t affect the function of the gun, or purchase a new mechanical box (P/N 870T-71) and transfer my upgraded internal parts into it. Since this particular gun endured quite a bit of abuse during my experimentation phases, I ended up purchasing an entire extra shotgun and transferring my upgraded parts over. On a related note, TAPP Airsoft have hinted at the possibility of releasing a reinforced mechanical box shell at some point. This would be a huge help to those of us who want to make our shotguns as durable as possible.


This addendum isn’t all doom and gloom. Despite the frustration caused by the mechanical box and transfer bar breakages, I don’t regret pursuing this project. There’s just something about using a manually-operated gun that is immensely enjoyable. This intangible quality is the reason you get “in the zone” more than when using a standard (semi)automatic. It’s something I personally enjoy quite a bit.

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9 hours ago, renegadecow said:

I just finished reading the google docs link you sent and was wondering how you were going to disseminate all that information. Was going to suggest breaking it into parts and make it a video series on YouTube.

I can't be arsed to record a bunch of videos, honestly. I plan on posting it here, and only here. Maybe it'll get the forum a bit more traffic.


I'll eventually remove links to the google doc so I can keep any updates all in one place.

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  • 2 weeks later...
14 hours ago, aznriptide859 said:



FYI now offers CNC'ed pump arms if you were looking for those. I have a pair in my TM breacher and they were a nice replacement (albeit not including the curved bit on the bottom).

While I'm sure the TAPP breacher pump arms are very high quality, they are fitted to the pump shuttle (where the pump handle itself is mounted) via one bolt each. That makes me nervous. I'm sure someone at TM calculated the shear forces involved with running the gun and determined that 2 bolts per arm were necessary.

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  • 1 year later...

Addendum #3: Troubleshooting low muzzle velocity with my top-folder


My previous addendum was incomplete, in that I failed to mention an ongoing issue with my second shotgun build. For some reason, I was unable to get the muzzle velocity to exceed 1J regardless of supplied pressure. The gun was functional, but not exceptional like my first shotgun build.



A list of the possible culprits for my issue.


The troubleshooting process took quite a bit of time, exacerbated by COVID-19 escalating my already demanding work schedule. I tried everything listed in the photo above, as well as completely redoing my hopup shims. These measures had no effect on the performance of the gun.


Frustrated, I tried the only thing I had left: manually bedding-in the hammer rails inside the mechanical box. I suspected that the trigger spring that launched itself into the mechanical box during my initial upgrade installation may have caused more damage than was immediately apparent. There was no visible deformation on the hammer rails, and the hammer itself seemed to slide freely when installed, which made this procedure a bit of a long shot.

On 9/20/2019 at 2:48 PM, ardrummer292 said:

Addendum #1: Shotgun number 2 complete!


Keep an eye on your trigger pin (P/N 870T-45) when removing or installing your trigger group. Mine slipped out of the trigger group during reinstallation, which led to the trigger spring (P/N 870T-44) launching itself into the inside of the mechanical box. While function-checking my gun, I noticed some peculiar jamming issues. Nothing seemed out of order until I noticed that the trigger itself had no spring tension on it. I removed the trigger group, fished out the (now mangled) trigger spring, and installed a Blackcat replacement. Thankfully, no additional damage was caused by the wayward spring wedging its way into the working bits of the gun. I added a small dab of Loctite to each end of the trigger pin in order to prevent this same issue from happening again.


Manually bedding sliding surfaces isn't a terribly difficult task. I disassembled the shotgun and removed all the internals from the mechanical box. Then, I applied pressure to the hammer and vigorously ran it over the hammer rails for a few minutes. I did this for each half of the mechanical box. After this was complete, I used Q-tips to clean up the plastic dust that had accumulated on the hammer and inside the mechanical box. After applying some GunSav grease, I reassembled everything and braced for yet another disappointment.


I wasn't expecting a miracle, but that's exactly what I got. Bedding-in the hammer rails did exactly what I hoped it would do. The gun puts out 1.5J using 0.40g bbs at 78 psi input pressure, which is fairly close to the 72 psi needed for my first shotgun build. I did a quick range test, which showed astounding performance. Using 0.40g bbs in 3-shot mode yields a measured and confirmed 220 feet (!) of effective range, without any need for hold-over. I ran out of 0.36g bbs, so I was unable to get a feel for effective range when using 6-shot mode.


My motivation for finally completing this troubleshooting is a bit unfortunate. Due to reduced game attendance (thanks again, COVID-19), I've done a serious reevaluation of my airsoft collection. I have decided to slim down my arsenal significantly, keeping only those items that I use regularly. Since my shotguns rarely saw field use - never, in the case of my top-folder - a solid half of my guns are up for sale.

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On 1/17/2021 at 5:07 PM, Wingmann said:

If you want to keep entertained, I have two, one Marui and one Double Whatever, that leak.

I can help you troubleshoot the source of the leak, if you like. I haven’t had to fix any leaks of my own, but I’m intimately familiar with the potential sources.

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  • 1 year later...

I've been back and forth to this thread the past couple years and I finally got my full-size 870 put together largely inspired by you man! I'll attempt to fine-tune the hopup, but for now the tighter spread at range I get with the full-length inner barrel set from WiiTech seems like a good upgrade! This weekend I'll be fielding it for the first time. I'll post a picture to show you after but I wanted to thank you! I'll let you know how the simple mods I've done affect my gameplay


I also have a question. I've been told that to chrono a tri-shot, you can just load a shell with one bb and regardless of the fact that the other two barrels are empty, the loaded one still shoots at the same power. That doesn't sound right to me, how have you been measuring?

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