Jump to content
CatgutViolin

GHK AUG H-Bar LMG w/ GBBR hicap

Recommended Posts

YxQS8WA.jpg

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:

tlA3qoz.jpg

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.

ZOt2BMI.jpg

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.

CHL4BE1.jpg

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:

N1EOeya.jpg

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:

yEFmWNh.jpg

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:

IzjcDxU.jpg

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.

nKyeU6y.jpg

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.

  • Like 8

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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

Share this post


Link to post
Share on other sites
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
  • Like 1

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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
  • Like 1

Share this post


Link to post
Share on other sites

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.

  • Like 2

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

×

Important Information

By using this site, you agree to our Terms of Use and the use of session cookies.