I know I’m not alone in having printed an AR-15 lower and test fitting it with internals – this fellow printed an upper to go with his printed lower, and another Thingiverse user just printed an AR-10 lower! I’d be pretty hesitant to use a printed lower with something as powerful as .308 (hence why I’m starting with .22), but I am impressed that a bulked up AR-10 lower can still be printed on something the size of a Prusa Mendel. I’m sure many others have also printed AR-15 lowers, but I can’t find any indication of anyone having actually fired one. I’m sure my printed lower will hold up just fine, though the response of many firearms owners is essentially “You’ll shoot your eye out, kid.”
Before I can put my money where my mouth is, however, I need to actually have a complete upper receiver. This weekend I finally got around to attaching the CMMG pistol length barrel that I have to an upper that I purchased many years ago. I’m not sure why CMMG decided to stake the front sight/gas block in place when it needs to be removed anyhow to attach a barrel nut, but I managed to drive the retaining pins out of the gas block, remove it, slip a barrel nut in place and re-attach the gas block. Why am I going through this trouble? Because due to the quirks of US law, a receiver can be switched back and forth between rifle and pistol configurations only if the first incarnation of the receiver assembled into a complete gun was as a pistol. I don’t want to limit myself, so the printed lower will begin life as a pistol in order to comply.
This subject of the upper receiver brings up another point – people have asked me if the upper could be printed as well, and I’m not nearly as confident of such a part as I am of a printed lower. When installing the barrel to the upper receiver, I found that the minimum barrel nut torque is defined as 30 ft-lbs (with a maximum of 80 ft-lbs allowed when ‘timing’ the barrel nut so that the gas tube will align in one of the notches on the barrel nut). I really doubt that an unreinforced thermoplastic can take up to 80 ft-lbs of torque on 1.25″-18 threads, especially given all the discontinuities present in a printed part. It’s probably sufficient to use less torque, as the barrel nut simply keeps the barrel attached to the upper receiver (and I believe the Bushmaster Carbon-15 uppers, which are a carbon reinforced polymer, specify a lower torque). All of the force from the shot fired is held between the bolt lugs and matching faces on the barrel extension, not between the barrel nut and upper receiver.
Assuming you had printed an upper receiver and didn’t overtorque the barrel nut, it would probably work fine. For a little while, at least. The problem with the AR-15 and its derivatives is that the gun ‘craps where it eats’. Many modern rifles are gas operated, meaning that they divert some of the hot expanding gases from the barrel to actually recock the gun (as opposed to being recoil or blowback operated). The AK-47 and AR-15 are both gas operated, but the Kalashnikov has the hot gases acting on a piston very near to where the gas has exited a tiny cross-drilled hole in the barrel. The piston is connected to the bolt carrier, and every time the gun is fired, gas pressure on the piston pushes the bolt carrier back, cycling the gun. In the AR-15, the gas is directed through a long tube all the way from the hole in the barrel right up to a ‘gas key’ attached to the top of the bolt carrier. This allows for much less reciprocating mass (which means that the AR-15 has much lower felt recoil than its Russian counterpart), but with the disadvantage that all of those hot gases (and other crud that comes from burning gunpowder) are blown right into the chamber above fresh rounds in the magazine – hence, ‘craps where it eats’. Since FDM style 3D printers use thermoplastics as a feedstock, these hot gases will undoubtedly start melting a printed upper. In fact, I’ve heard reports of reinforced polymer uppers starting to melt after repeated rapid fire. Fortunately, piston systems are becoming more widespread on the AR-15 platform, which would eliminate the ‘hot gas melting the upper’ issue, but I’d still be hesitant to try using a 3D printed upper even for just rimfire cartridges – reinforcement would be needed, I think.
Since I’m using a CMMG .22 kit, it doesn’t need a buffer and buffer spring (which is great, as I don’t have those parts anyhow). In fact, it doesn’t need anything attached to the rear of the lower receiver at all, but I wanted to have something in place to help provide support for the ‘buffer tower’ (the ‘loop’ at the top rear of the lower receiver). More importantly, I wanted an excuse to finally use the nice 1-2″ thread pitch micrometer that I bought several years ago.
I stuck a piece of 1.25″ scrap aluminum rod in the lathe, and turned some threads onto it.
When the micrometer indicated I was getting close, I threaded on an actual aluminum lower to test for fit. Afterwards, I opted to fit out the lower with internals as well, as I figured it was prudent to test the untested upper and .22 conversion with a ‘proper’ aluminum lower first.
This morning I hunted around for ammunition, which took me a good 20 minutes (while I am a firearms enthusiast, I don’t think I’ve fired more than a dozen rounds or so in the past 5 years). After realizing that I had no .22 ammo (yet discovered cartridges for guns that I do not own), I made a stop at the manliest store on the planet to pick some up (if Bruce Campbell were a store, he’d be Fleet Farm). I then headed to a top secret testing facility (Dad’s farmland) and carefully assembled the upper onto the aluminum lower. Absolutely nothing had been previously tested, and this was actually the very first AR-15 I’ve assembled (or even owned), so it was with a fair bit of trepidation that I loaded a magazine into the gun (with only a single round – always test unproven systems with a single round to begin with). After cocking it and carefully letting the bolt forward to chamber the round, everything looked to be in place, so I aimed (as well as one can ‘aim’ with nothing attached to a flattop upper) 20 feet away into the dirt and fired. Everything worked fine, so I reloaded with 2 rounds and repeated, followed by 3 rounds. All systems functional!
I switched out the lower for my printed version and double checked the operation. Would it hold up? Again, one round in the magazine, cock the gun, squeeze the trigger, and… Wouldn’t you know it, I shot my eye out. Just kidding – it functioned perfectly. Testing again with 2 rounds, then 3 rounds, then a full magazine. Everything ran just as it should, magazine after magazine. To be honest, it was acting more reliably than a number of other .22 pistols I’ve shot. I ran close to 100 rounds through the gun before getting annoyed with not actually being able to aim at anything, and decided to call the experiment an overwhelming success.
To the best of my knowledge, this is the first 3D printed firearm (as per the definition in the GCA) in the world to actually be tested. However, I have a very hard time believing that it actually is. My Stratasys is a good 15 years old, and Duke Snider’s original AR-15 CAD files have been floating around on the ‘net since early 2000. As such, I can’t imagine that I’m the first person stupid adventurous enough to actually pull the trigger on a 3D printed receiver. If someone has beaten me to it, please leave a comment!