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GHK AUG H-Bar LMG w/ GBBR hicap


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I didn't make a build log before completing it as I had no idea whether it was actually going to work out, but here's a conversion of the GHK AUG to replicate the real AUG LMG variant used by the Austrian military and... nobody else, as far as I can tell.

The real LMG consisted of basically just a normal AUG, but with two changes: A 24" barrel with integral bipod, and (doctrinally) 42rd magazines rather than the normal 30. There was a drop-in trigger pack that converted the gun to fire from an open bolt, but for all intents and purposes, the AUG LMG is basically just a different barrel and a permission slip to use extended mags and full auto.

So, there were really two parts to this: Building the 24" barrel setup, and building a hicap magazine so that it could actually be used in an LMG role for airsoft purposes.

First, the barrel:

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Unfortunately, I could not find an airsoft replica of the original H-Bar bipod, and I wasn't going to sacrifice a real one for this, so I settled on a modern Ratworx bipod. This is an original design based on a combination of the AUG bipod and the Steyr SSG 08 bipod, and while not completely accurate to the original, I think it still looks the part (and works quite well, too).

To get the 24" barrel length, I used a combination of barrel extensions. In the process, I discovered that GHK's 20" barrel is actually a bit short. Since I have a real AUG to refer to, this was rather puzzling, since they both appear to have the same length- I eventually realized that on the GHK ones, the locking nut that secures the flash hider is actually cast as part of the flash hider, so the barrel is a bit shorter to compensate. So, I ended up using about 5" of barrel extension.

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From front to back:

-Angel Custom AUG H-Bar flash hider (intended for AEGs, but it's standard 14mm negative, so works fine)

-Ratworx bipod locking nut and Ratworx bipod, over a 2" barrel extension with the outer diameter reduced from ~18mm to 17mm. I don't have a lathe, so this was a royal pain in the *albatross*. Once I reduced the diameter sufficiently for the bipod and locking nut to fit, I drilled into the barrel extension and tapped the hole for a small grub screw, which hits a shelf on the bipod about in the middle and stops it from sliding back further.

-1" barrel extension

-2" barrel extension, with another grub screw at the rear. The grub screw is there because of an issue I found with the bipod- the locking nut incorporates a grub screw that acts as a travel stop, so that the bipod only has about 45 degrees of rotation to either side. When the gun is set on the ground, it falls over, the travel stop hits the bipod, and the remaining momentum (or even just weight) was sufficient to start unscrewing any part of the barrel or barrel extension that relied on a friction fit. So, all the barrel extension parts are epoxied together, and then the combined extension unit is secured to the barrel threads with that grub screw, in the same manner as the cheesy plastic flash hiders that come pinned to most AEGs.

So, that's the bipod. Relatively straightforward. Now the magazine.

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This is a hybrid of the top 2/3 of a GHK mag mated to the lower 2/3 of a TM mag, with an ugly seam where I had to resculpt part of the joint due to a chop saw-induced mishap. It's easier to understand what's going on if I overlay the internals over the shell:

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So, at the top, the GHK magazine internals have been cut very short. The original gas reservoirs had threaded caps on the bottom, so I had the brilliant idea to measure the threading, order an appropriate tap, and tap the remaining stub of the front reservoir to insert the original plug. Unfortunately, while the threads are definitely 17mm diameter and 1mm thread pitch, the threading cut by an M17x1 tap was too shallow, and inserting the plug ground off its threads. So, it was epoxied in place, and a grub screw was inserted from either side to provide additional support and then ground flush.

I then drilled through the middle of the plug, tapped it for 1/8" NPT, and inserted a 45 degree elbow followed by a 6mm push-connect adapter. Only the front gas reservoir actually leads to the valve, so that was the gas system sorted, but I still had to make more changes to the GHK part of the mag. For starters, I needed to secure the mag to the shell, since normally GHK mag internals are held on at the baseplate. This was easy; I just drilled a hole for a cross pin, cut a nail to length, and peened the end slightly to get a good friction fit in the shell.

The other issue was reloading, since obviously a hicap normally fills from the top. This was my solution:

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I drilled out the fill valve threading to 6mm, countersunk a larger hole, and inserted a 6mm ID O-ring which stays via a friction fit. The hole on the back of the shell is sized for a speedloader, so the mag is reloaded by inserting a speedloader and then just shoving BBs through the O-ring. They drop into what used to be the rear gas reservoir, which opens into the body of the magazine. Simple.

At the bottom of the mag, I cut down the TM hicap internals as pictured, and epoxied a length of spring tubing to the hicap's feed track. The hicap internals secure to the baseplate:

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So, no further modification was needed.

To get the spring tubing to fit through the feed track on the GHK side, however, I had to run a 8mm drill bit up from the bottom and down through the top. This was a bit nerve-wracking as it would have been very easy to chew through the front of the mag.After all the internal work was done, I cut a pair of 1" x 4" plates from some 1/8" steel sheet, and epoxied them in place to connect the two mags.

Unfortunately, I had accidentally mangled the top of the TM mag when cutting it on my chop saw, as the plastic used on it was much more brittle than the GHK mag and it chipped. So, I had to do a bit of reconstructive surgery at the joint, which I did using a mixture of steel-reinforced JB Weld epoxy putty (for strength) and Kneadatite hobby epoxy (for sculptability) in about a 2:1 ratio.

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After that was fixed and all the test fitting went well, I spray painted the whole thing. The join is noticeable up close, but I was more concerned about durability, so I think it worked out alright in the end.

The final hicap holds about 450 BBs, and can feed 250-300 on a single full wind, so should be usable for an LMG role.

Because the barrels are quick-swappable, and the HPA conversion was done to the magazine, I can swap from gas-in-mag rifle to HPA light machine gun in just a few seconds:

 

Next on the agenda is making at least one more hicap, and modifying an Odin speedloader to work with these. With a competent A-gunner, I'm thinking even two magazines should be enough to maintain constant fire, with the A-gunner reloading and rewinding one mag while I shoot the other.

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Work continues, so now I'm going to document it as I go.

While the H-Bar functionality is complete, I am still experimenting with hopup. The GHK AUG is peculiar in that its stock hopup unit is designed to accept AEG-type buckings and barrels, rather than the VSR-type as is common with GBBs. However, the T-NT aftermarket hop unit is a more conventional GBB type. Since I ordered a spare barrel to make the 24", I have two stock hop-up setups, so I could mess with one and leave the other alone.

So, I've done a bit of experimenting thus far and found some very peculiar results.

Stock hop-up setup: Reasonably consistent hop, decent accuracy, occasional horizontal deviation.

T-NT setup (w/ T-NT barrel and bucking): More consistent than the stock setup, both vertically and horizontally, but not a huge difference.

Stock hop-up unit, R-hopped 6.05mm PDI barrel from a reputable source, flat-hopped Lonex bucking, pink eraser nub: Very consistent horizontal dispersion, but completely inconsistent amount of hop applied.

Weird, right?

Well, with some experimentation I think I've found the culprit- a while back I remember reading about similar behavior with Polarstar guns, specifically caused by the very rapid action of the solenoid-driven nozzle. Essentially, the nozzle was propelling the BB into the hop unit with such force that it was seating the BB inconsistently on the hop-up. This inconsistency led to underhopping, overhopping, hooking, and generally accuracy. It seems reasonable that this behavior would apply to GBBs as well.

I calibrated each hop-up to the .32s I use, then removed them from the barrels and tried manually pushing a BB through each. The stock setup and T-NT setup both required some force to get past the contact patch in the bucking. The R-hopped barrel, on the other hand, offered less resistance, but across a longer distance. As well, the Lonex bucking is considerably softer than the stock GHK bucking, and the T-NT one (the GBB type) is the stiffest.

So, it seems like the GHK and T-NT hops have enough resistance to stop the BB in a consistent location, while the R-hop is allowing the BB to seat at varying 'depths' and thus cause problems. However, the flat shape of the R-hop provides better horizontal consistency than the traditional contact patch on the GHK and T-NT buckings, so the R-hop still has better horizontal consistency. I think this explains why we haven't seen much in the way of R-hop or flat hop type buckings for GBBs.

Interestingly, with Polarstars the recommended solution is to tighten the bucking lips, but I found no noticeable difference when trying this on either the GHK or T-NT setups. In fact, the GHK and T-NT bucking lips are very loose and provide no resistance to the BB at all (not a problem for sealing, since the nozzle doesn't actually seal on the bucking, but rather the entire hop unit), while the Lonex in the R-hopped setup does provide resistance at the bucking lips.

With all that noted, my next step is to order a PDI W-hold bucking- which I've anecdotally heard works well in the GHKs- and give that a go with the PDI barrel sans R-hop patch.

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Quite simple solution to the R-hop/flat hop issue:  Lighten the recoil spring so the BB doesn't get thrusted into the chamber at high velocity leading to inconsistent seating.

 

Nice work on the bipod, though my biggest concern is the barrel rigidity.  The GHK stock barrels were just aluminium so it already has rather dubious rigidity, and now the bipod is there.  Even on the real steyrs, the weight of an M203 unit is often enough to shift the zero of the rifle.

 

I would probably use a steel barrel and then use an alu barrel extension.  Threads won't do so the best way is to get the barrels to overlap the inside and outside sections to a really fine dimension and then pressed, glued and pinned to get a tight fit.

Edited by 3vi1-D4n
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11 hours ago, 3vi1-D4n said:

Quite simple solution to the R-hop/flat hop issue:  Lighten the recoil spring so the BB doesn't get thrusted into the chamber at high velocity leading to inconsistent seating.

 

Nice work on the bipod, though my biggest concern is the barrel rigidity.  The GHK stock barrels were just aluminium so it already has rather dubious rigidity, and now the bipod is there.  Even on the real steyrs, the weight of an M203 unit is often enough to shift the zero of the rifle.

 

I would probably use a steel barrel and then use an alu barrel extension.  Threads won't do so the best way is to get the barrels to overlap the inside and outside sections to a really fine dimension and then pressed, glued and pinned to get a tight fit.

Adjusting the recoil spring is a possibility. Unfortunately, the recoil springs are contained within telescoping guide rods inside the stock, so adjusting them is a bit of a pain, and they're already light enough that it generally won't go into battery with less than half of its stroke.

Definitely agreed regarding barrel rigidity. While I bought the steel gas block for the rifle barrel, on the LMG one I'd much prefer for the gas block to give out rather than break the barrel if something gets smacked. The Ratworx bipod is actually quite light (it's aluminum), but I do worry a bit about it catching on something. My issue is that, as far as I can tell, nobody makes a steel AUG barrel, and I'm not sure whether a barrel from an AR or something could be retrofitted to work. In theory, I suppose as long as I can secure the gas block in place, that should allow it to lock into the receiver; then it's just a matter of fitting the hop unit.

Edit: Well I'll be damned, Hephaestus makes a steel barrel! Got one ordered; I'll combine it with the steel gas block to make a 'main' barrel and swap LMG extension / rifle flash hider as needed, and then I can use the aluminum combo as a backup.

Edited by CatgutViolin
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I have lightened my recoil spring on my Steyr to 80%, seems fine.  But yeah it does depend on your internal friction.  RS Steyrs by design are pretty bad when it comes to managing friction, so they are both overgassed, and over sprung.  The GHK is actually much better when it comes to that, as it has a nearly contactless bolt carrier to the receiver, and the hammer springs are rather light.

 

In terms of accuracy, I modified the hop unit so it would fit a modified VSR hop rubber, on its AEG barrel, which I modified to fit the VSR hop rubber.  That seems to work fine, and saves me buying a TNT hop unit.  

 

I find that the GHK stock rubbers are great as they are, until it overwhelms with silicone oil or water, then they have < 100ft of range.  VSR rubbers are a bit more tolerant for water, and a little better for oils, but still oils do affect it.   So having the VSR hop already is an advantage.  With a VSR hop unit you can change rubbers to flat hop types, though it does depend on your knowledge on tuning. 

 

I find that flat-hops don't work if you don't have the right nub in the hop units.   Soft hop nubs and hop rubbers give better hopping range and consistency, but leaks gas/low FPS and doesn't hop heavy BBs.  Harder hop rubbers give higher FPS, can use heavy BBs but doesn't give consistent hops, and then the shape of the hop rubbers make a difference too.  

 

Anyways, keen to see how the Steyr HBAR project goes.  I would never use a bipoded Steyr per se, but its still cool to see something unique.  I personally want to make a WE RPK, but that would require a GHK receiver which I am not willing to spend money on just yet.

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Minor update: The PDI W-hold bucking, using the PDI barrel without the R-hop patch, works surprisingly well. The W-hold gimmick actually seems to work for keeping the BB in place and producing a consistent hop effect, and after a minor initial break-in period I am getting great consistency, both horizontally and vertically, producing a ~6" group across five shots using HPA 0.32g BBs at 100ft. I think this might now actually be the most accurate airsoft gun I own inside of 100ft, although I'm not sure how it will perform at longer range.

Now I'm just waiting on the steel barrel, and extra TM mags so I can convert more hicaps, and then this rifle/LMG is good to go.

Edited by CatgutViolin
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  • 4 weeks later...

After a couple weeks of waiting, the gun itself is done.

OiMbQH2.jpg

I also added an RMR mount from Shapeways, so as to have a backup red dot for quicker acquisition at short range.

The Haephaestus steel barrel is very nicely made. It didn't fit the barrel quite as tightly, so I had to shim it a bit, but otherwise it was easy to fit the gas block and get it all reassembled.

Not really much more to say. The gun works and the barrels interchange. With the steel gas block and barrel, the gun actually is only half a pound lighter than the real thing, and the balance is identical. I'm looking forward to putting this through its paces.

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On 7/6/2020 at 9:12 PM, CatgutViolin said:

After a couple weeks of waiting, the gun itself is done.

Beautiful.

Out of interest, could you try (or have you tried) fitting the GHK magazine internals into a RS shell? I'm losing hope on GHK ever making translucent shells and am contemplating trying to fit a RS one. The BB track could be a separate issue if the reservoir fits inside the Steyr shell.

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  • 2 weeks later...
On 7/9/2020 at 7:44 AM, PureSilver said:

Beautiful.

Out of interest, could you try (or have you tried) fitting the GHK magazine internals into a RS shell? I'm losing hope on GHK ever making translucent shells and am contemplating trying to fit a RS one. The BB track could be a separate issue if the reservoir fits inside the Steyr shell.

(Sorry for the late reply)

That was my original intent, as I hopes it would be easier to just shove the GHK internals into the real 42rd shell.

Unfortunately, the GHK magazines have a wider taper at the front than the real ones, and the real shell has ribs to guide the follower which would need to be removed. Ultimately I judged it easier to do the splice, if that gives some indication as to how difficult it would be.

Edited by CatgutViolin
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  • 5 months later...
Posted (edited)

Well, I haven't gotten around to making more hicaps or even using the thing in-game (thanks, pandemic!), but I was reminded the other day that the open-bolt trigger pack was a thing for AUG LMGs and figured hell, why not give it a try.

Does it actually make the gun better? Hell no, it makes it minutely worse, since now it has lock time. Why'd I do it?

1. The real one had it and my relentless obsession with 'getting it right' won't leave me alone, so this is closer to how it should be.

2. More importantly, being open-bolt and full-auto-only are both pretty major distinctions from a normal AUG. Since all the fields I play at are semi-only unless you have a replica of a real-world LMG, not only does this make it a more accurate replica, but it gets me further away from 'I put a long barrel on my AUG and brought a hicap, gib full auto pls'.

Anyways, the way it works is pretty straightforward.

  • I removed the semi-auto sear and ground the catch lugs off the hammer. The autosear is the only thing holding the hammer back.
  • I replaced the normal bolt catch with a homemade one, and made a long linkage to connect to it. When the trigger sear moves back, the linkage depresses the bolt catch, releasing the bolt.
  • When the bolt fully reaches battery, it trips the autosear to drop the hammer and fire the gun.

So- all the trigger does now is release the bolt, and releasing the bolt is all that's needed to fire the gun. This means the trigger pack doesn't have a functional bolt catch, but since I wasn't planning to ever use 30rd GHK mags in the LMG configuration, I really don't care.

No modification was made to the gun itself, so it's all of ten seconds to swap back to a normal trigger pack. I've got a set of parts on the way to build another trigger pack so I can swap between them. Worst case, if the open-bolt trigger pack fails in-game- not all that unlikely given the Khyber Pass standards I built it to- I can swap to the normal pack and get back in play.

On another note, this AUG is robust. For a GBB it doesn't have a lot of moving parts, everything in the trigger group is chunky and overbuilt, and at ~3k rounds I see very little noticeable wear to any of the parts. It is free from the tightly fitting parts with tiny critical surfaces that are the bane of my existence on WE guns. One of my tests involved repeated full auto 500rd bursts to test air consumption, and it took it like a champ.

So, on that note, I wanted to test a question: On a gun that shoots too hot normally, what's more efficient- using a grub screw adjustable nozzle to bring the power down, or running it at a lower input pressure? So I figured out what pressure my green gas gets me (~105PSI), dialed in my air compressor to that level, and saw how many shots it took to get a 10 PSI pressure drop from full. Then, I fully backed out the grub screw (maximizing power), and reduced the pressure until I got it shooting at the same muzzle velocity as before (at around 72 PSI). I repeated the test, and this time it got about twice as many shots.

The takeaway here is that nozzle obstruction, while it does reduce muzzle velocity, absolutely sucks for gas efficiency. I hypothesize that with the grub screw method, pressure builds behind the screw, which once the rocket valve closes and the BB has left the barrel is then bled out as waste. It also opposes the closing of the rocket valve, delaying the whole system and potentially wasting more air. The better way to fix it is to adjust the rocket valve to either close more quickly and/or allow less air through, but GHK *fruitcage* frustratingly helpfully glues together the nozzles so that they cannot be disassembled. I've thus ordered both an unassembled nozzle from Redwolf and another assembled '1J' nozzle (read: 1J through a 16" barrel if you use .2s, MUCH more through a 20" barrel with .32s) that I intend to butcher for parts and modify further, and hopefully will end up with a nozzle that does not need to be restricted to produce field-legal FPS on green gas. Then the real fun part will be seeing how that compares, efficiency-wise, to the high-power nozzle at low pressure. Either way, I expect it to cycle much faster than the somewhat sluggish rate in the video below.

Oh yeah, and it turns out the Shapeways mount has enough flex that it loses zero (to a significant degree, even by airsoft standards) if I bump it. Which, given how the RMR protrudes out the top like a battleship's conning tower, will be about thirty seconds into a woods game. Thus I have elected to swap to a CNCed equivalent from Samoon, if it ever shows up.

 

Edited by CatgutViolin
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Apologies for barging in on your thread, but I figured it would be minimally less disruptive than having to make a whole new Pictures thread, since those have all been archived.

Further apologies for the terrible photography, but your GHK HBAR is no longer a lonely soul!

PrNqeDh.jpg

Incidentally, if you want a very-slightly-messed with steel gas block at a big discount, buzz me.

 

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Awesome gun, and I'm perpetually envious of that Hexagon H-Bar kit. Is it an AEG, or did you adapt the barrel setup to fit the GHK? Also sending a PM your way about the gas block. Edit: I just found your post on Reddit about it being a GHK, using a cut-down Hephaestus barrel with a variety of extensions- that's a really clever solution! Have you run into my issue where the bipod caused the H-Bar extension to rotate off, or is there something that secures the extension?

Small update- I got the Samoon rail accessory, and while I was correct in my assumption that the RMR base was a separate piece, it's actually the big rail section that holds it on. I don't use any gucci accessories so that big (IMO ugly) rail doesn't need to exist. A cutoff wheel solved that pretty quickly and now I have a nice CNCed RMR mount.

I also discovered something very interesting: Samoon didn't send me a low-power nozzle last year. They sent me a high-power nozzle, and with it coming in an unmarked bag I had no way of knowing. So it seems I've been trying to get field-legal FPS using a nozzle intended for 450+fps in a shorter barrel with lighter BBs, so no wonder I had to obstruct most of the nozzle to get it under 1.5J. The assembled standard-power nozzle is still shooting significantly hot (~370 with .32s), so I'm going to test further with the low-power nozzle and see what I can get. I'm hoping that the low-power nozzle should get me under 1.5J, and then either the grub screw mod or lighter BBs (through the 14" barrel) will let me drop it down to 1J for CQB.

Also, I'd been experimenting with using a flat-hopped Madbull Shark bucking, but started noticing occasional shots above or below the typical FPS, so I examined the barrel and discovered that the lip had torn. Unsurprisingly, the very soft rubber of the bucking couldn't stand up to the violence of the GBBR action, so it's back to the PDI for now. I also suspect, since at the moment I'm testing in full-auto only while I wait for the new trigger parts to show up, that the precise timings involved in full-auto are less kind to weaker seals. So, I'm going to do some comparative testing between the PDI and T-NT to see how they each do on consistency; I have a feeling most AEG buckings won't seal as well as a thick GBB bucking. It looks like the T-NT will work with VSR buckings with the side tab cut off, so I might experiment further there as well. I might just end up using the PDI setup for a rifle setup, and the T-NT for the MG setup, where durability and consistency will have to trump accuracy.

Edited by CatgutViolin
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5 hours ago, CatgutViolin said:

Awesome gun, and I'm perpetually envious of that Hexagon H-Bar kit. Is it an AEG, or did you adapt the barrel setup to fit the GHK? Also sending a PM your way about the gas block. Edit: I just found your post on Reddit about it being a GHK, using a cut-down Hephaestus barrel with a variety of extensions- that's a really clever solution! Have you run into my issue where the bipod caused the H-Bar extension to rotate off, or is there something that secures the extension?

I have indeed run into exactly that issue. I could in theory drill and tap it for a locking screw (to prevent it rotating) but it’s a rare part and that’s an ugly modification... Obviously the desirable approach would be to cut down another Hephaestus barrel to use for the Commando/regular configurations, and then use a proper grade of Loctite to secure the HBAR to this Hephaestus (which it’s timed to).

That way I could complete a proper assembly with a longer inner barrel etc., but that’s all dependent on my being able to get another steel gas block for the second assembly, which as per my PM might not be as easy as I’d hoped...

I’m also watching your hop-up tests with great interest, since I have a pricey Black Widow/Maple Leaf combo in mine with precisely zero shots (cheers, COVID) through it at present.

Edited by PureSilver
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Yep, drill-and-tap was my ugly solution. My plan is to (eventually) get a second full steel barrel setup, Loctite the extension, use the grub screw to secure it further, and then fill the grub screw hole with steel-reinforced epoxy and paint to match the barrel. I should think that'll be pretty solid.

I did a bit more hop testing today, actually, and two things came of it. First is that the PDI bucking is slowly getting mangled as well and started to leak, so at this point I have very little confidence in using AEG buckings in the stock hop unit and will be retiring it. Second is that, since I want to test more than just the T-TNT, I went ahead and ordered that same Black Widow hop unit as you, as well as a Maple Leaf MR-hop for it to try.

I tested the nozzles and found that the 'normal' one is shooting 370 with .32s as described above, but the 1J one is actually shooting 1J. So, I opened up the normal nozzle again (since this is the one I assembled, rather than buying a glued pre-assembled one), cut down the rocket valve spring, and closed it up again. Now it's doing 300 with .32s, which is perfect, but with some occasional spikes to as high as 330. I'm also seeing significantly higher inconsistency than before with my known good nozzle,  which makes me suspect the full-auto operation is creating inconsistency and the modified rocket valve needs to bed in. Either way, I'm going to wait until I get my replacement trigger pack (I've been testing by HK-slapping to drop the bolt and fire the gun- not exactly optimal!) to test/evaluate further.

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Man, this has been a day. I was going to wait to post again until I got my new parts and could properly test, but I have blown the last three hours trying to figure out the WEIRDEST problem.

Okay, so, I now have three nozzles. Previously, I was using a high-power nozzle (~370-380fps on propane when unrestricted), with a grub screw mod to restrict the aperture and bring it down to 300fps. HPA was producing 300fps at ~105PSI, so comparable performance at comparable pressure.

I modified the normal-power nozzle to do about 300FPS with .32s on propane. When I plugged in my HPA mag at ~105PSI, it was noticeably more sluggish and outputting about 250FPS.

I immediately assumed there was a problem with either my regulator or the mag. I tested the reg with other guns. I swapped the gasket on the mag. I swapped the output valve. I started questioning my life choices.

Then I tried the low-power nozzle. It was doing 260FPS with propane, and 240FPS on HPA. Much less of a discrepancy.

Then, finally, I tried my original configuration with the restricted nozzle... and lo and behold, suddenly it worked as it originally did. 300FPS from both power sources. What?

Here's my hypothesis.

Normally, when people HPA tap GBBs, they replace the inlet valve with a QD connector. So, the original reservoir is retained; it's just constantly being topped off by the external line.

On my setup, however, there is no internal reservoir- the only 'onboard' gas is just whatever fits in the tiny headspace area of the magazine. So when the gun fires, it's almost entirely with air being supplied from the external line, and the throughput is garbage. This means lower FPS for a given pressure.

So why does the high-power nozzle give me the expected FPS? It seems to be the grub screw- as I said in my Jan 8 post, it creates backpressure in between the grub screw and rocket valve. I think what's going on is that the backpressure delays the rocket valve closing, allowing the relatively slow throughput of the regulator to still build significant pressure inside the nozzle before the rocket valve seals and the blowback cycle begins.

So. The bad news is my original plan- set up a nozzle to do 300FPS on .32s with little to no restriction on both propane and HPA- is physically impossible. The good news is my previous configuration will still work fine, and now I have additional options. I'm seeing it looking like this:

-CQB: Low-power with propane, or normal-power with HPA at 120PSI.

-Field, rifle configuration: Normal-power with propane, or high-power (heavily restricted) with either propane or HPA at 105PSI. The former will be more efficient with propane, but the latter allows HPA as an option.

-Field, LMG configuration: High-power, unrestricted, with HPA at 72PSI. This provides field-legal FPS while keeping low pressure to reduce wear and tear.

That's my TED Talk thanks for coming. I'm going to go drink.

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  • 2 weeks later...

Minor update. I made two more hicaps. It went surprisingly quickly.

I then tried painting all my mags in a more FDE color. After some deliberation I decided I didn't like the outcome, so I got out my can of acetone and tested on a discreet area of a GHK mag. It held up fine, so I applied acetone to strip the paint.

When I pulled the hicaps out of the acetone bath and noticed that the lower halves had turned to goo, it occurred to me that the TM mags probably aren't made of the same space-age polymers as GHK.

So. I broke the mags in half, discarded the TM halves, and thought about how to rebuild them. Since I was going to have to buy more mag shells anyways, I figured might as well hack up some of my GHK mag shells. This way I could mate GHK to GHK and just get the TM internals to fit, rather than lining up the dimensionally-different TM and GHK mags. And what do you know, this turned out to be tremendously easier. There's less of a seam, the TM internals and baseplates fit the GHKs with minimal modification (pretty much just tearing the feed track out of the GHK shells), and the end result is cleaner and stronger.

MSjaPYu.jpg?1

Unfortunately the paint did some screwy stuff and dried with a bit of unwanted texture this time around, but at this point I really cannot be bothered to redo them yet again, so I'll live with it.

Oh yeah, and I made one more tweak to the mag design.

hmytFnx.jpg?1

Basically instead of using an O-ring to retain the BBs, which requires me to use a speedloader to reload a hicap (not ideal), I made a small tab out of spring steel. The tab can slide upwards to allow loading, then slide downwards to retain the BBs, and friction keeps it in place.

Now just need to do some hop testing and this project is tentatively finished. I'm leaving open the possibility that I might make more mags, depending on how it turns out the gas efficiency fares versus how easy they are to reload.

Edited by CatgutViolin
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While range is something that I feel I can assess pretty well just from observation, accuracy is a lot harder to gauge once it gets better than 'reliably hit a man at 100ft'.

So, today I did some objective testing of accuracy on targets set up at 20yds. Don't read too much into the placement of groupings relative to the 'X', as they used different outer barrels and may not have been aligned the same. There was no holdover; all shots were fired with the rifle braced and the 'donut of death' reticle centered on the X.

Test candidates:

  • Stock hop unit, stock hop bucking, stock barrel.
  • Stock hop unit, PDI W-Hold bucking, PDI 6.05mm barrel.
  • GBL hop unit, GBL bucking, Maple Leaf barrel.
  • T-NT hop unit, T-NT bucking, T-NT barrel.

mkULbGM.jpg

Observations:

  • The GBL unit + bucking with the Maple Leaf barrel was disappointing, having the worst group of all of them. I am not sure if it might work better with lighter BBs, but the solid adjustment arm (that doesn't use a nub) produces inconsistent hop with .32s.
  • I tried the same setup with a Maple Leaf MR-Hop, in case the bucking was the issue, but that was so inconsistent that I didn't even bother to set up the target. In a GBBR, the MR-Hop needs a soft nub to function consistently; with the hard arm the bucking exerted so little pressure on the BB that I was running into the inconsistent chambering problem again.
  • The T-NT hop was better than the GBL/ML. Not the tightest grouping, but it hops well and is reasonably consistent.
  • The stock setup... isn't bad, actually, having a tighter grouping than either of the GBB-style setups. However, it struggled to hop the .32s; while consistent, it didn't seem to have sufficient backspin to stabilize out to longer ranges.
  • And lastly, while the PDI setup had two fliers at the start that did not repeat upon subsequent testing (I'm going to chalk it up to contamination, either in the barrel or on a BB), it demonstrated the tightest spread among the remaining BBs, beating the stock setup, and also demonstrating the most consistent hop and farthest range.
  • Both the GBB buckings seal better on the nozzle than the AEG buckings do.

Conclusions:

  • The stock hop-up is accurate, but is much better for lighter BBs than trying to hop heavy ones a great distance. It struggles past 150ft or so, and the arc is more parabolic than flat.
  • The GBB-style units seal better on the nozzle, and will almost certainly stand up better to wear and tear. They're also better-built (CNCed aluminum, rather than plastic).
  • While I was not impressed with the GBL setup, the T-NT one hops heavy BBs well and produces very consistent muzzle velocity, albeit with greater spread than the stock setup.
  • The PDI W-Hold is the best mix of range and accuracy.

Takeaways:

  • I'll be leaving the stock setup in my 14" barrel for CQB. Accuracy is handy in CQB to hit small exposed targets, and I don't need range.
  • The T-NT is the better of the two GBB units, and since it can be ordered in preassembled form, is the better drop-in upgrade. I may test with a Crazy Jet barrel at some point, in case it is more accurate. I'll be using this in the 24" LMG setup, where I'm more concerned with longevity than accuracy.
  • The PDI W-Hold remains the best bucking I've tested. Just swapping the stock bucking for the W-Hold is the cheapest and most cost-effective upgrade available to this gun. This will be going in a 20" rifle setup.

tl;dr Buy a PDI W-Hold for a whopping $10 USD and you are good to go.

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Oh yeah, and through judging the pressure drop across a 1000rd burst (80 seconds of continuous full-auto) hooked up to my pancake compressor I determined that I should expect ~6,000 shots from a full 90CI/4500PSI tank, or ~3,000 from a 68CI/3000PSI tank. More mags might be in the cards as I did not expect it to be that efficient, nor to handle the 1000rd burst with no noticeable wear or damage.

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But Wait, There's More! I feel like the more time I spend on this gun, the less I understand GBBs. This is turning into a full-on GBBR research project.

So the T-NT setup is what I'd been doing most of my testing on to date. It produces a consistent ~300fps in the LMG configuration, on HPA, with an unrestricted nozzle.

I tried the PDI. It was getting 260fps. Weird. Seems like an air leak.

I swapped to my nozzle intended for green gas/propane, which is partially restricted by a grub screw, and filled a mag. Now the T-NT was doing 310, and the PDI was doing 300. The gap between the two was almost gone. So clearly not an air leak.

I swapped back to HPA, and tightened the grub screw, obstructing the nozzle about 1/4.

The T-NT setup went up in power, now to 320-330. I tried the PDI, and it was also achieving 320-330. Further tightening the grub screw then reduced muzzle velocity and rate of fire as expected.

Total speculation: The PDI setup may require more force to get past the contact patch compared to the T-NT, since it projects out farther. This may mean that the PDI takes longer to leave the barrel, so hasn't traveled as far when the rocket valve cuts the gas flow. So, restricting the nozzle creates backpressure which delays the rocket valve closing, which in turn means it accelerates the BB for longer, increasing muzzle velocity on both but particularly the PDI. This implies there is a sweet spot for nozzle cross-sectional area, striking a balance between how quickly the rocket valve closes and the throughput of the nozzle.

GHK GBBRs aren't especially unique so these principles must apply to other guns as well, but I've never seen any of this so much as mentioned before. Maybe it's just the fact that I have the ability to play with all these different variables that is revealing how they interact with one another.

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Got a real-steel surplus stock. Looked it over, took some measurements, decided it's not worth the effort to try to convert to airsoft use. Too many differences, and I'm petty and don't like the purple-brown color I got.

Also got a real-steel gas block. This was MUCH closer to GHK spec, but had the slight problem of still containing the stub of the barrel within it (like this). Worse, the stub is interference fit, so it is basically impossible to remove non-destructively.

Non-destructively. Hah. Austrian precision engineering stands no chance against an American caveman with power tools and brute force. I used a cutoff wheel to cut a channel along the length of the gas tube, turning it into a C-shape, then walloped the barrel stub out with a mallet. I slipped the gas block over my spare Hephaestus barrel, epoxied it in place, and filled in the seam where I cut the tube. Voila, another full steel barrel setup.

The fitment isn't quite perfect, even after a bit more alteration with a Dremel- you can see it's a bit misaligned, and the charging handle doesn't quite go all the way forward. But it's fully functional, so works for me. I transferred over the gas piston parts from the GHK barrel and it's good to go. It ended up being cheaper than the now-discontinued Samoon gas block was.

46uGz66.jpg

Bw3jbRJ.jpg

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8 hours ago, FELDJAGER said:

Just chiming in to say this is a super cool project and great to see so much testing of this gun. 

 

Whats the verdict on the ghk aug in general? seems pretty damn robust.

I like it a lot. I've owned a bunch of GBBs and a couple of GBBRs over the years and this one is by far the easiest to work on and the best performing out of the box. Recoil is on par with some of the Daytonas, the hop-up works and has lots of aftermarket support, it's quite efficient, and all the functional components (in particular the massively oversized trigger mechanism parts) have shown no significant wear after ~4,000rds fired.

It's also a GHK, so while mags are expensive, you have the option to DevilHunter mod them (ie convert to fill with liquid CO2 from a bottle), which helps a lot in cooler climates. Although probably not necessary in SoCal.

The gun does have some quirks, the most significant being the inexplicable decision to use a magnet instead of a spring for the bolt catch, which means out of the box it often fails to disengage when the charging handle is operated. On mine I added an extra neodymium magnet, and that bit of increased power seems to have solved it.

Also the stock aluminum gas block is fragile and will shear if you torque the foregrip too hard. Hence my steel replacement.

But that's about it. It just works, and replacement parts are readily available if need be. Really the biggest problem is just that a basic rifleman load of gun + 6 mags will run you about $675.

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  • 2 weeks later...

Time for an update. First up, I recorded the process of making more mags (hopefully this should resolve into an embedded video if/when Youtube responds to my appeal, thanks automated flagging)

I am now up to five hicap mags, for ~2500-2750 BBs before I need to reload. That seems fine for a machine gunner. Based on shop shop compressor testing, I believe I should be able to get ~8,000 shots out of a full 98/4500 tank. I was extremely skeptical of that level of efficiency until I decided to see just how many shots I get out of a green gas mag, and got 110 before it stopped being able to lock back. So that's cool.

Next, I found an issue with my GBL hop-up from the accuracy test: the crown of the barrel was actually bent slightly. I swapped it out for a Crazy Jet I had lying around and the accuracy is significantly better. The actual amount of hop is a bit inconsistent, but I suspect it will need to break in given that it doesn't use a nub so is much more sensitive about amount of applied hop.

On top of that, a bizarre thing happened- I've discovered that the 'high-power' nozzle I've been using, which when I got it was shooting ~1.7J unrestricted and prompted me to use the grub screw mod to reduce it to 1.45J, has settled down to ~1.45J even when the grub screw is removed. It's consistent across different barrel setups, and given the efficiency mentioned above I am not inclined to think it's an air leak. To test I also procured another standard nozzle, and it also was shooting ~1.7-1.8J out of the box. After a hundred rounds or so, it started shooting anywhere from 1.4J to 1.7J. Now it's mostly shooting ~1.45J, with occasional spikes, mostly at the start of shooting- with .32s it'll go like 370, 350, 355, 340, 300, 305, 300, 303, 320, 300, and so on. So I guess there must be some kind of break-in period on the nozzles, which I really can't explain.

Well, I should be ready to go at this point, whenever I can safely attend a game.

RZcuRzZ.jpg?1

Edited by CatgutViolin
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