Gunsmithing with a 3D printer – Part 3

To say that the buzz generated around this project is heavy on “media hype” would be an understatement.  I could write a great deal on this alone, but I’ll content myself to refer people to David Chernicoff’s excellent article explaining why this is not a big deal and the apocalypse is not nigh.  Being at the center of a story really lets one see how the media sausage is made, and I’m amazed at how much misinformation gets copied and introduced as a story gets picked up by a string of outlets.  It’s like a giant journalistic game of “telephone”.  The past few weeks have also seen a far bit of buzz on the Defense Distributed project, which aims to design a 100% 3D printable firearm.  It’s certainly an interesting engineering challenge, and one which I’ve pondered myself over the past year and a half.  The problem is that even the strongest 3D printable thermoplastic currently available for the FDM process (Ultem 9085) doesn’t even have half the tensile strength needed to withstand the 24000 psi maximum allowed chamber pressure of the .22LR round as defined by SAAMI.  As such, yes, a 100% 3D printed gun made on a RepRap could certainly go ‘bang’, but even with a barrel of large enough diameter to keep it from exploding, there would be so much deformation in the bore that most of the available energy would be sapped by gas leakage around the projectile (to say nothing of the utter lack of accuracy).  In the end, you’d have a smoking, charred crater left for a barrel bore after the single shot.  Quite an expensive proposition, given that such a gun would almost undoubtedly be classified as an AOW, requiring sign-off by a chief law enforcement officer, background check, submission of fingerprint cards, $200 for the tax stamp, and up to a 6 month wait for approval before you could commence printing one.  If you have an interest in hobbyist gunsmithing, make sure to familiarize yourself with the rules and regulations that your project would have to abide by – it’s not worth risking a paid vacation to ‘Club Fed’ to 3D print a ‘zip gun’ that could very well cause a great deal of injury to yourself and others.  Please stay safe and legal, everyone.

On a more interesting historical note, I found that my printed lower is not in fact the first 3D printed firearm to be tested (as per the GCA definition, where the receiver itself is legally a firearm).  Many people pointed me to the Magpul Masada, as the prototypes had SLS printed lowers and furniture.  However, the lower of the Masada is not the controlled part – it is in fact the upper receiver, which was machined aluminum on the prototypes.  No, the first tested 3D printed firearm as best I can tell was actually a silencer!  Yes, as per the definitions of the 1968 GCA, a silencer is by itself considered a firearm.  Admittedly, this starts splitting hairs, and there may very well be other examples of prior art – Magpul’s FMG-9 prototype was primarily built with SLS printed parts, but used a modified Glock 17 as the core, and I’m unsure of whether the receiver was Glock or SLS.  In fact, it may very well be that exactly what constitutes the receiver on the FMG-9 has yet to be decided – there has only been a single prototype made, and until the ATF’s Firearms Technology Branch is asked to determine which is the controlled part, it could be entirely unknown.  As well, firearms companies have been incredibly secretive about their usage of rapid prototyping (I’m still trying to track down specifics on the SLA silencer) – I imagine there’s some engineer out there saying “Boring!  I did this stuff like 10 years ago!” but can’t say a word due to non-disclosure agreements.

Anyhow, back to tinkering.  While the tests on the printed lower ran just fine with .22 ammunition, the real test would of course be the round that the AR-15 was designed for, the .223 Remington cartridge.  I re-assembled my original DPMS 20″ bull barrel upper and attached a collapsible stock to my printed lower.

Again, with a fair bit of trepidation (though tempered with an engineering background), I used only a single round to begin with, which functioned just fine.  A much louder report than .22LR to be sure, but I was pleasantly surprised by the utter lack of recoil – Eugene Stoner was a very sharp fellow, and despite my misgivings about a direct impingement system versus a piston based system, I’m impressed by how effectively his design works.  However, when adding more rounds to the magazine in testing, I had issues with extraction and feeding.

I switched out my printed lower for my aluminum lower and tried again.  To my chagrin, the problems persisted, so I stopped testing, wondering if perhaps the steel-cased ammo I was using could be to blame.  The fact that I still didn’t have a detent for the rear takedown pin was also bothering me, as it meant that I didn’t yet have a fully functioning 3D printed lower (and as things loosen up and wear in, the rear takedown pin tends to drop out onto the floor without the detent in place).  I purchased some 1/8″ OD brass tubing with an ID suitable for the detent spring from McMaster-Carr and set about machining an insert that would house the spring and detent.

I did have to drill out the front of the tube slightly, as the detent is a little larger in diameter than the spring itself.  I also tapped the rear of the tube for 4-40 threads so that a set screw would keep the spring in place without any need for an end plate (so the lower can be operated as a .22 pistol with absolutely nothing screwed into the buffer tower).

After drilling out the hole in the lower to 1/8″, I pressed in my machined detent tube (with set screw, spring and detent) with a dab of solvent to secure it in order to capture the tube in the lower receiver.  It would have been nice if Stoner would have made the lower receiver so that it didn’t require such work, but realistically, an AR-15 stock would rarely (if ever) need removal (in fact, proper assembly procedure is to stake the rear plate in place after the castle nut is tightened).

I then gave the upper a good cleaning and oiling – while it was still brand new, the fact that I had purchased it a good 6 years ago meant that it was extremely dry.  I also purchased some brass .223 ammunition, as some uppers just don’t like steel cased ammo, and I wanted to improve my chances as much as possible.  Testing with the brass cartridges and freshly cleaned upper yielded excellent results with the aluminum lower, with perfect cycling.  Swapping in my printed lower, however, brought the old feed and extraction issues right back.  So, what could be the issue?  My primary suspect is flex in the buffer tower.

There is a small gap between the upper and lower, and this gap does indeed widen as the rifle is cocked due to the increasing force from the action spring located in the buffer tube.  Without a spring installed, the gap is about .027″, and with the spring installed, the gap is about .034″.  Pulling the charging handle all the way back widens the gap to .040″.  As such, the buffer tube actually gets flexed downward when the BCG (bolt/carrier group – the primary reciprocating components in the rifle) is moved to the rear during the firing cycle.  Since the BCG actually slides into the buffer tube, keeping the tube and the upper receiver axes aligned is critical, and binding results from this flex, causing the feed and extraction issues.  I decided to do a bit of rough FEA (Finite Element Analysis – computer simulation of the actual bending) in SolidWorks to see how well it matched what I was actually seeing on the printed part.

I used the default parameters for ABS and applied a rearward force of 15 pounds (the approximate force I measured with a fish scale needed to begin moving the BCG rearward) to see what the calculated deformation would be.  As it turned out, the model says that the buffer tower should actually be bending about 0.011″ rather than the .007″ I was seeing, and that was with the stock ABS values, not values that would better represent the weaker 3D printed part (as opposed to something injection molded from the same material).  I think the buffer tube and end plate themselves provide the extra rigidity that real-world measurements are showing, and I’ll have to see how I can best simulate their addition.

Meanwhile, I know that the buffer tower is not as large as it should be – the new ATI Omni lower is bulked up even more than my version on both the buffer tower and front takedown lugs.  As a side note, my front takedown lugs have cracked once more where the layers had originally split, so my current design is not sufficiently robust in that area either.  Bulking up my lower’s buffer tower to a similar state as the ATI lower shows that the tower would bend only about .008″ in the simulation.  However, even that may not be sufficiently rigid.  Commercial polymer lowers are not made of ABS, but are instead a glass filled Nylon 66, which is far stronger.  Even using unfilled Nylon 6/10 in the simulation brought the flex down to only about a quarter of that of ABS – still close to an order of magnitude more bendy than aluminum, but probably in the range of reliable functionality.

As such, I think the best way to use a 3D printed AR-15 lower with .223 is to better support the buffer tube from underneath.  Oryhara has done precisely that with his thumbhole buttstock design.  While he’s only fired it so far with a .22 upper, I’m guessing he’ll have much better operation with .223 than I have.  In the meantime, I’ll try applying a bit of carbon fiber to the buffer tower (and front lugs) on my printed lower and see if the feed and extraction demons can be tamed somewhat.