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’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.