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:
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.
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.
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:
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:
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:
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.
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.