Well, to say that a lot has happened since Part 3 would be something of an understatement.  To recap:

Defense Distributed tried printing my AR lower STL on an Objet machine, but it only held up for 6 shots.  While much was made in the media about this ‘failure’, I thought it was actually an excellent demonstration of material properties in the two different 3D printing technologies used.  My FDM printed lower used material with a higher impact strength, while the Objet printed lower was stiffer.  As a result, mine flexed and would not cycle properly with .223 ammunition, while Defense Distributed’s lower cycled perfectly with 5.7x28mm ammunition, but fractured at the root of a buffer tube thread (interesting how the extreme detail afforded by the Objet process actually created stress risers due to the threads in my STL model being perfect ‘V’ profiles with no filleting of the thread roots).  They’ve since refined the model to hold up for 600+ rounds, which is quite impressive for a photopolymer.

Representative Steve Israel started calling for renewal of the Undetectable Firearms Act, and has also called for making 3D printed firearms and homebuilt ‘undetectable’ polymer magazines illegal.  I’m taking this somewhat personally, as he keeps using a giant photo of my AR lower as .22 pistol during his press conferences – the least he could do is put my URL on the photo to provide proper attribution.  Given that some of the most popular rifle magazines commercially made today are of polymer construction, I’m not sure what he’s really hoping to accomplish by expanding a law that was passed due to media hysteria over the introduction of the ‘plastic’ Glock pistol.  Wait, media hysteria over plastic guns?  The more things change, the more they stay the same…

After the horrific shooting in Connecticut, Thingiverse pulled almost all of the firearm related files, including my AR lower and a Magpul style trigger guard I had designed.  I immediately contacted their legal counsel and pointed out that a trigger guard is a rather important safety device and has use in paintball and airsoft, not just firearms.  The response was ‘our sandbox, our rules, and we can change the rules at any time’ (but spoken in far more lawyerly terms).  An AR-15 grip that had also been taken down was reinstated a few days later, so I’ve been asking how to get my trigger guard reinstated as well.  However, my requests appear to be ignored, and I’m somewhat giving up on Thingiverse at this point.  It’s still a great community, but when I can’t use it to share with other gunsmithing hobbyists or even paintball and airsoft enthusiasts, my desire to use it naturally diminishes.  Meanwhile, Thingiverse appears to have no issues with people sharing drug paraphernalia designs, so maybe they’re attempting to cater to a rather different group of ‘hobbyists’.

For anyone interested, I have a copy of my original AR lower STL here (though I don’t really recommend it at this point – there are much better 3D printable lowers that have been designed and refined by other folks).  I have a copy of the trigger guard here.  It comes in two versions – one is the standard version that uses a roll pin through the rear holes, and the other I designed to be a tool-free version that uses angled studs to snap into place.  I’m actually rather proud of this version, and would be happy to hear feedback on it.

Back to the present – I really haven’t done anything further with the printed AR lower, as I’ve been experimenting with a different firearm platform.  Commenter Allen had asked “Could the Ruger 10/22 receiver be built the same way?”  This certainly got me wondering, as the 10/22 receiver, unlike an AR-15 lower receiver, is what the barrel attaches to, and contains the reciprocating bolt.  Additionally, the fire control group (trigger, hammer, etc.) is contained in a modular pack rather than having those components fitted individually to the receiver.  Plus, answering this question seemed like an excellent excuse to finally purchase a 10/22 – like the AR-15, it’s an incredibly popular rifle with countless aftermarket accessories available.  Additionally, it’s a great platform to learn the fundamentals of proper marksmanship (one of the many skills that I’d like to learn one of these days).

I found a very well used one at Gander Mountain for a reasonable price – the sling swivels had apparently broken off long ago and the receiver finish was a bit worn, but it looked to be in good functional order and would do well for learning how the rifle operates and is constructed.  When I got it home, I eagerly dug into the disassembly to see how it functioned and to give it a much needed cleaning.  The 10/22 is a semiautomatic, blowback operated .22 rifle.  The blowback operation means that unlike the AR-15, the bolt is not locked into place when the gun is fired and is only kept forward by means of the recoil spring.  A blowback bolt is also quite heavy in comparison to the cartridge used – this is to ensure that the bolt begins its rearward travel in the firing cycle slowly enough to let the chamber pressure decrease to a safe level before the spent cartridge is extracted.  The receiver itself is cast, though there are aftermarket billet receivers available for those looking to heavily customize the rifle.  In looking at how the bolt reciprocates in the receiver, it appeared that there should be no issues with a 3D printed receiver, provided that the print is made so that the layers are parallel to the barrel axis (to provide as smooth a surface as possible for the bolt’s travel).

For the printed receiver itself, I again turned to Justin Halford’s cncguns.com for an IGES file.  Unlike the AR-15 lower receiver, there weren’t any design features that I felt needed strengthening right away, so I created an STL file directly from the solid model and set it running with the same Bolson black ABS I had used for the AR lower.  I printed the receiver upside down so that the interior didn’t need any support material, and thus would provide as good a finish as possible.

After removing all evidence of support material, it was time to start fitting parts.  Chief among these is the barrel itself, but the hole in the receiver was slightly undersized (not unexpected, and better than being oversized).  I clamped it to the angle plate on the mill and indicated it in vertically with a dial indicator and coaxially with a Blake co-ax indicator before opening up the hole with the boring head.

After bringing the hole to appropriate size (I could just begin to insert the barrel shank), I tapped the barrel clamp holes with 12-24 threads (thanks to the blueprints at fireamfiles.com) as well as the stock mounting screw hole in the front tang.

Next was to actually test installation of the barrel itself, which tightened up nicely, but I noticed that the barrel would visibly cant downward as I tightened the clamp screws.

After removing the barrel, I saw that there wasn’t quite enough clearance on the front counterbore, and the back of the barrel was catching on the top front edge of the receiver.  So it was back to the boring head to enlarge the diameter on the mating face slightly.

Then, it was time for a test fit of the bolt – it was a tight squeeze to get it past the rail on the inside right of the receiver.

As it turned out, I think the rear wall of the receiver in the original IGES file may be a touch too thick, as I also couldn’t get the trigger pack installed, so I thinned out the rear by perhaps 0.030″ until I could just get the retainer pins to go through the receiver and trigger pack (I had already reamed out the holes in the receiver at this point).

With that done, I could finally fit all the internals and actually dry fire the gun.

However, when I tried to drop the receiver assembly into the wood stock, it wouldn’t fully seat.  After fumbling with it for a few minutes, I noticed that there is an extra relief cut on the original receiver at the interface between the tang and the receiver front.  As it turned out, the original IGES file does indeed have this relief cut, but when I brought it into SolidWorks, I had run a feature recognition pass on the part.  For some reason, SolidWorks removed this feature – I should have just done a direct export to an STL file instead!  Oh well, one last machining pass on the mill took care of it.

The barreled action fit just fine in the stock, and both the 10 and 25 round Ruger magazines fit, though perhaps a little more loosely than desired.

Today I took it to the range and found an accomplice to act as a model.  Naturally, I let him burn through some rounds on the 3D printed AR receiver configured as .22 pistol first.  A .22 AR pistol is kind of a ridiculous contraption, but it is also ridiculously fun.

Next was the test of the printed 10/22 receiver.  As with previous testing, I started with only 1 round in the magazine and worked my way up.  Things were running just fine, so I put in the 25 round magazine and let ‘Secret Agent Man’ have some trigger time with it.

Generally, it ran nicely, though we did have some feed issues with it.  I think the fitment of the magazine could be to blame, as it seems that the front of the magazine is able to tip down a little too far.  Both magazines are also absolutely brand new (this was their first usage), and I’ve been told that 10/22 magazines operate better after an initial break-in period.

So there you have it – a 3D printed 10/22 receiver is entirely feasible!

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.

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!

I’ve used my Stratasys to prototype out various ideas for paintball gun parts, but the concept of using it for actual firearm parts hadn’t really occurred to me until early last year.  I first thought of making some dummy 12 gauge shells to test out the action on a Remington 870, and then thought of using it to test out 1911 pistol grip panel ideas.  Gun manufacturers have been using rapid prototyping for years, and the concept is now making its way to the hobbyist gunsmith.  To the best of my knowledge, this has been restricted to mockups (Justin Halford used a stereolithography made frame to test component fit for his fantastic Beretta 92FS project) or less critical parts like furniture (grips, buttstocks and such). It wasn’t until I came across an AR-15 magazine follower on Thingiverse that I began to wonder about the feasibility of making more functional parts with a rapid prototyper.

The use of plastics in firearms is a relatively recent development as far as primary structural components go.  Firearms have certainly used plastics early on (the use of phenolic ‘Bakelite’ was popular for grips and other previously wood furniture in the years leading up to WWII and well afterwards), but use of plastics for a core component took much longer.  Consider a car analogy – we’ve seen plastic dashboards for many decades, but the use of plastic for something as critical as an engine block wasn’t attempted until the early 1980s.  It wasn’t until 1959 that Remington (at the time owned by DuPont, hence having access to cutting edge polymer technology) came out with a .22 rifle that used plastic for the receiver (the core ‘body’ of the gun).  This was the Nylon 66, so-called since the Zytel-101 material used was a type of Nylon 6-6 polymer.  While it was quite a popular rifle (selling over a million units by the time it was discontinued in 1991), and helped further the use of synthetic stocks among shooters, it wasn’t until Glock pistols became popular that polymer firearm frames/receivers gained widespread acceptance.  Today, polymer framed pistols outsell their metallic counterparts, and new rifle designs increasingly use molded synthetic receivers.

The AR-15 rifle, while designed to use an aluminum lower receiver, has such limited force imparted while firing that I guessed it could probably be made of printed plastic with little worry of breakage.  After all, Orion’s Hammer has successfully made a lower from HDPE (after having limited success making one from a pine board), not to mention the commercially produced polymer receivers such as Bushmaster’s Carbon 15 and Plum Crazy C-15. It would easily fit within the build volume of the Stratasys, but my concern was whether or not it would have enough precision for all features to be usable (Orion’s Hammer didn’t worry about the takedown pin detents or bolt catch, for example).  Rather than waste a lot of material on a failed idea, I took Justin Halford’s IGES file of the lower, scaled it to 75% of full size, and set it running with PP3DP filament.  The resulting print looked fantastic:

Figuring that my chances with a full scale print were excellent, I decided to modify the model by strengthening two areas that I was slightly concerned about – the front takedown pin lugs and the bolt hold catch lugs.  Adding more material to the model in SolidWorks was pretty straightforward, and I finished it up by adding an integral trigger guard.  I switched out the PP3DP filament for some black Bolson ABS – after all, the ‘black rifle’ would look a bit odd in ivory (more importantly, it’s easier to see/photograph detail on dark material).  After slicing the STL file, I sent it to the Stratasys and waited a few days (no speed demons, these old machines).

After breaking away all of the most easily removed support material, I had a great looking print.  I had generated the STL file at a very high resolution, as I was wondering how well the buffer tube screw threads would actually turn out (having not yet tried printing any threaded objects).  As it happened, perfect!  A buffer tube screwed right into the threads with no cleanup required.  Naturally, I wanted to share my results, but unfortunately firearms are presently a bit of a touchy subject.

The concept of using a 3D printer to manufacture gun parts has not been lost on the RepRap community, and the topic has been debated a number of times on the RepRap forums.  At this point, there is a policy proposal to not allow weapon designs or projects to be uploaded to the RepRap library, and a line on the Health and Safety page for the RepRap project states “the RepRap researchers will work actively to inhibit and to subvert the use of RepRap for weapons production” (emphasis mine).  On the other hand, Thingiverse once had a rule against weapons in their terms of service, but later removed that restriction.  Afterwards, the Thingiverse upload page still said “Please don’t upload weapons. The world has plenty of weapons already,” but I assumed that this text was not updated after the TOS was revised.

I decided to ask for clarification on the Thingiverse mailing list.  The phrase “kicking the hornets’ nest” aptly describes the resulting discussion, I think.  In the end, Zach ‘Hoeken’ Smith (one of the Thingiverse founders) weighed in and clarified that such content is allowed, though discouraged. Fair enough. Apparently someone had taken notice of the commotion, and three weeks later, there was an STL file of a lower receiver posted to Thingiverse in what could be described as a confrontational manner.  Since the cat was out of the bag, I decided to upload my own STL model, as I wanted to hear constructive feedback on how the version might be improved to better suit the current limitations of 3D printing.  Well, apparently the resulting ‘weapons on Thingiverse’ debate raged hard enough that in February the lawyers were unleashed upon the site’s Terms of Use, and now uploading any content that “…contributes to the creation of weapons…” is verboten. Although that policy doesn’t appear to be enforced, I suppose they could yank my uploads and kill my account at any time, hence I’m re-documenting my work here.  Enough rabble-rousing – back to the fun stuff.

I’m rather jealous of people who can print the lower receiver with soluble support, as clearing support material from small diameter holes is a bit of a pain.  I used a pin vise and an assortment of small diameter drill bits to clear out all the long cross drilled holes in the part, using Duke Snider’s receiver blueprint for dimension references.  With all traces of gray polystyrene eradicated, I set about cleaning up the larger holes, as they were ever so slightly undersized (better than being oversized).  I ran a 5/32″ drill bit through the holes for the trigger and hammer pins, and eagerly installed the fire control group.  The trigger and hammer  functioned flawlessly, with no slop apparent in the pins.  The selector lever was a bit of a tight fit, so I worked it back and forth perhaps a hundred times to break it in.  After tapping the 1/4-28 thread for the grip screw, I attached the grip, keeping the selector in place by virtue of its detent.  Similarly, the magazine catch was a bit of a tight fit, and I had to carefully work the part back and forth in the receiver to make sure that it would reliably retract under force from the magazine release spring.  I then ran a 1/4″ drill bit through the holes for the front and rear takedown pins.  Unfortunately, I heard a quiet snap when drilling out the front hole, and sure enough, there was a break between layers.

On the plus side, this confirmed my suspicion that the takedown lugs needed reinforcement in the first place.  I brushed on a bit of Weld-On 3 to fuse the layers together (delicately, recalling what happened when I dunked printed parts in MEK).  After running a drill bit through once more, the cleanup was complete, and I installed the takedown pin with its spring and detent.

Nice!  Now, for the other area that had given me concern – the bolt hold lugs.  Sure enough, when I pressed in the roll pin, I had layer separation.

Well, I never cared much for roll pins anyhow – they always seemed rather brutal (especially when driven into a blind hole – yikes).  After touching up the damage with a few more dabs of Weld-On 3, I ran a 3/32″ drill bit through the hole.  I then threw away the roll pin and instead used a dowel pin of the same size.

A little bit of superglue on either end of the pin should suffice to keep it in place.  Finally, there was the rear takedown pin to contend with.  Justin’s model appears to have the recess for the pin head as around 5/16″ or so, while the head on the pin from my DPMS parts kit measures 3/8″.  No worries – I lightly clamped the receiver in the mill vise, centered the spindle over the hole, and carefully widened the counterbore out with a 3/8″ endmill.

After this, the takedown pin fit perfectly.  Since I don’t actually have a full AR-15 stock (and will be attempting to run this receiver as a pistol first), I needed a way to capture the detent spring for the rear takedown pin.  I opted to tap 4-40 threads in the rear of the spring hole and kept the detent and spring in place with a 1/8″ long 4-40 set screw.  Unfortunately, the force on the detent was heavy enough that when I tried to slide the takedown pin into the receiver, the detent broke through the thin wall into the rear of the FCG area.  It appears that extra 1/8″ of spring compression due to the set screw may be too much.

I dabbed on a bit of ye olde Weld-On 3 and clipped 1/8″ off of the spring to compensate before attempting to secure the pin again, but the detent still wanted to break through the wall.  I’ll leave it out for the time being, but I’m considering drilling the hole out larger and sleeving it with brass tubing.

Overall, it’s looking quite promising.  The upper receiver fits snugly, and magazines can be inserted and removed with ease – shown is the lower with an upper attached along with a .22 magazine that I intend to use with the CMMG .22 conversion kit.