Tokyo Marui M870: building the ultimate airsoft shotgun

[ardrummer292]

On 5/25/2022 at 11:50 PM, PayloadCo_Blazes said:

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?

Sorry @PayloadCo_Blazes, just seeing this now. I'm glad you found my writeup useful. Going "whole hog" with a build like this is a process, but it's fun to push the performance envelope of a system to see what it can really do when the work is put in.

 

Chronoing tri-shots is tricky. In a perfect world, loading a shell with one bb would give you a clean, realistic velocity reading. In reality, the other two barrels being empty absolutely effects the flow and pressure within the system, affecting the reading you get from the one loaded barrel. I've found that chronoing the shotgun when loaded as normal provides decent results. You're likely to get the odd crazy value (way too high or way too low), but I think it's safe to discard those since the chrono is likely picking up the entrance of one bb and the exit of another.

[Jacob h]

Hello, recently got myself a tm m870 it shoots pretty great tbh. It’s second hand with some “upgrades” but weren’t specified.  I’m thinking of getting all this wiltec upgrade parts you reference.  Little intimidated by the process but it’s worth a try.  I’m currently hpa tapping the stock using an adapter.  Got the tank and all.

What would you recommend for my first purchases of parts?   Also what should I get for maintenance?   Reading this has clued me in that I might need to actually maintain my m870. Which I’ve never done. So that’ll be a long and daunting task. But oh well 

 

thanks 

Jacob h

[ardrummer292]

13 hours ago, Jacob h said:

Hello, recently got myself a tm m870 it shoots pretty great tbh. It’s second hand with some “upgrades” but weren’t specified.  I’m thinking of getting all this wiltec upgrade parts you reference.  Little intimidated by the process but it’s worth a try.  I’m currently hpa tapping the stock using an adapter.  Got the tank and all.

What would you recommend for my first purchases of parts?   Also what should I get for maintenance?   Reading this has clued me in that I might need to actually maintain my m870. Which I’ve never done. So that’ll be a long and daunting task. But oh well 

 

thanks 

Jacob h

Honestly, if it's running the way you want it to run, I'd leave it be for now. A parts breakage is a better reason to crack it open than a non-specific desire to "upgrade." If the shotgun is lacking in one specific area, that can inform your upgrade path. What do you want it to do better/differently?

 

I wouldn't worry about maintenance too much, since that will also involve disassembling the shotgun. When it comes time to take it to bits, you can handle installing upgrades and performing preventive maintenance all at once.

[PayloadCo_Blazes]

I totally forgot to post a picture of mine, realized when I came back to refresh myself on this stuff. I'll be doing maintenance soon and upgrading, but it's really held up! It's the one at the top. I've found that the real wood handguard sounds amazing when racking compared to many polymer ones I've used. The whole layout is simple, but very comfortable. My kit is really where I really made improvements this year.

 

The SRU grip on the breacher is very wobbly and the hopup doesn't work great in it. I'd love to build something very similar to your underfolder since I've had the S&T steel folding stock on my very first spring shotgun ever since I started and would love to immortalize it in some way. Yours is real pretty! The sawn-off was the only thing I bought new and it's really fantastic! I played an entire AMS event with just that and my pistol. I was surprised by how comfortable the wood grip is, and how solid the pump grip is, and I was really surprised by the range it has, despite having a wicked short barrel. I feel like Jag definitely improved the hopups of their newest lineup of shotguns and I'd really like to just drop one into my full-length 870 to see how it does before I try just shimming it like you described, that sounds like a terrible time honestly. The M26 is neat, but I would never use it in-game unless I was able to modify it to take CAM 870 CO2 shells. I've seen it done before, along with a tutorial for how to do it, but it was all in chinese and hard to understand even with pictures. But it still bugs me to think of having a gun that I'm not using. If I were to do anything else, I think a KSG looks pretty sweet with a drum mag, and I'm holding out hope that the TM Saiga-12 breaks ground for future semi-auto shotguns like a Benelli M4. I'd love to do a Beretta 1301 (like the Langdon Tactical one from Garand Thumb).

[ardrummer292]

On 12/4/2022 at 7:34 PM, PayloadCo_Blazes said:

I'd love to build something very similar to your underfolder since I've had the S&T steel folding stock on my very first spring shotgun ever since I started and would love to immortalize it in some way. Yours is real pretty!

It was a super cool setup, but definitely not easy to complete. Finding a machinist willing to modify the stock-to-receiver interface was difficult, and the loss of the folding function (due to routing the HPA line through the hinge) was kind of a bummer.

[PayloadCo_Blazes]

I bet you could add the round pipe rotating piece on top of a breacher grip at the right height, that has the line routed through at 90 degrees already, and instead of having the whole pipe rotate with the arms, the arms just rotate on the pipe. You'd have to do something to to keep that motion from being metal on metal or it'll creak and wear like crazy, but I came up with a different solution:

Looks pretty sweet, right? I swapped the stock and grip on a Cybergun SLP for the S&T grip with the folding stock. I figured out a way to make it fit and I just need to tap it for a line and add a few bits to make the grip more comfortable. I bought the SLP just to mess around with and see how it actually is. IT IS TERRIBLE. Not bad on single-shot mode, but tri-shot is as you'd expect through a single barrel. If I get this working with HPA, I'll leave it on single shot mode and treat it like a slug round shotgun.

 

This is why I'm SO excited for the Saiga-12 and to see how it operates internally, because the idea of a semi-auto system that feeds with TM style shells is already great on paper (obviously not in practice the way Cybergun did it), but a semi-auto tri-barrel gun could really be a huge game-changer, especially since hearing TM pushed the release way back due to issues found in QC, with feeding I think. That speaks volumes for them as a company and makes me not as skeptical of the performance of the gun! The only thing I wish they had done was come up with a compact way to do individually adjustable hop ups, but I also wonder if there's a way to modify an AA-12 / SGR-12 with an HPA engine?