I want to know how difficult it will be to print and build a gun with a consumer FDM printer. I want to know if the gun will be any good. I want to share some of the information I’ve learned so far that has been helpful to others. I want to contribute something back in the process if I can, maybe making it easier to do or making the printed gun better.
I decided to go with an AR-15 style rifle. I might try other designs based on the outcome of this one. The image below shows how the lower receiver will be assembled (not my image or design). Finding design files (because I don’t want to design them from scratch… yet) was step one. It was a lot easier than I thought it would be.
There are a lot of really good designs out there and they’re not on the sites getting all of the attention in the news. This one was/is on GitHub. It’s part of a repository run by FOSSCAD.
Got the files, sweet. Next is the material. The printer I’m using doesn’t have an enclosure and I don’t really want to build one so ABS is probably not going to work. After some brief research PETG came out as the most likely candidate. It tolerates high temperatures reasonably well, doesn’t change size with temperature changes, and should be about the right amount of ductility. I ended up getting some Amazon Basics filament while it was still in stock. So far it has been very consistent in size and it arrived dry. Not bad considering the price.
I immediately ran into issues printing. I’ve been using ABS for a number of years and PLA a bit so this caught me off guard. PETG is very sticky apparently. The first layer kept peeling off the bed because it was catching the nozzle as it would come back over printed areas. Increasing the first layer height and slowing the printer waaaay down for the first layer fixed this issue. Once I got past that first layer issue the first print came out pretty good. I was worried about the threads for the buffer tube but they came out almost perfectly.
Over the last week I’ve been printing out the parts. The next step is to assemble the parts into a usable lower receiver but I’m waiting on some parts in the mail. When I get it assembled or run into issues doing so I plan to post an update.
If you have questions or comments please feel free to post but I’m going to avoid politics. For me this is about making something and learning something new.
Good luck with the build. For political reasons I’ve heard it is always smart to put a serial number on these parts rather than not to. This is often to deal with how authorities view your item, rather than it’s actual legality.
Also, I’ve heard the buffer area is most common breaking point. So consider your printing of that area.
There’s a version of the design where the buffer tube mount can be milled out of metal. I might end up going there at some point but I want to see how far it can be taken with consumer FDM printing first. The design is really beefed up in that area so I’m thinking it’ll work, at least for a little while.
Come on Bill, don’t be like that. No one challenges your manhood for not holding the rockets you build and hand lighting them. Don’t challenge people to do dumb things. Rather take the time to suggest best practices. I would suggest not hand firing this for your first firing. Components could break violently and you are still learning the stresses in your system.
I don’t think a string will be necessary though Nick has me wondering. I’ll look into options and do some research. The failure modes for a lower receiver don’t seem dangerous but maybe I’m missing something.
I’m not an expert, but I have seen a commercial polymer ar15 lower break at the buffer tube connection. This happened with the guy next to me at target master gun range about 2 years ago. The Ar failed to extract and then kicked the buffer tube and butt stock off the firearm. No one was hurt, but that seemed like luck given how close shooter’s face was to where the gun failed. It was an unwanted o’fuck moment at the range.
I know from personal experience that color affects ABS.
I suggest for the next purchase get filament without color. While PET-G itself has some wonderful properties, the stuff that makes it black may not share those properties.
Glue stick helps considerably with first-layer adhesion. Blue tape helps somewhat with first-layer adhesion.
I purchased some M3 hardware to assemble the bolt together lower receiver. Two things about this were unsatisfactory to me. One, I was having trouble finding matching hardware in stainless because M3 x 50mm is a very odd bolt size combined with the other required bolt sizes. Two, M3 is way tinier than I remembered it to be. I used to use M3 hardware a lot in high school while working at a hobby store but it didn’t seem so flimsy at the time.
Both of those things are not a big deal but I wanted to explore more options in addition to the bolt together lower receiver because… Well, because I can and I don’t need any other reason. I sliced and examined about a dozen lower receivers and decided to go with this one next.
The design seemed reasonable to print and the design seemed sufficiently sturdy. A lot of the designs simply didn’t have enough reinforcement for FDM printing, like military specification designs meant for aluminum with a logo stamped into them. I sliced it with some pretty strange settings relatively speaking, I think it had 10 perimeter loops or something like that and 50% gyroid infill. If I had to do it again it might do solid infill but all my research suggests that perimeter loops are more important than infill because of the extrusion direction. So 25 hours later this compelling print came out.
Holy shit, removing support material from PETG is difficult. It adheres to anything nearby like mad. After about an hour of removing support it looked like this. I’m pretty happy with it (assuming parts fit and it’s physically accurate).
The threads for the buffer tube are the only thing that concerns me. I have a tap coming in the mail from Amazon for like $30 to clean up the threads for the buffer tube. If that goes as planned I think this print will be a “success” for a 3D printed lower receiver.
The bottom looks good. The top looks the opposite of good. I used support for the print but it might not have been tuned correctly for this specific application.
Thanks for the link. Valid point about the string. It’s good to see this stuff being covered in a level headed way. It’s worth investigating further if you want to get to the bottom of it though. The barrel in that video was printed incorrectly.
And it was printed by a guy who runs a 3-d printing business, who (theoretically) should know how to tell it was done right or not. I have a feeling there are gonna be more than a few home 3d printer users who have a similar experience.
A 3D printed barrel design is a whole different game than a lower supporting a factory barrel with integral locking lugs. The only meaningful energy the lower and buffer tube support is the gas pressure derived energy that cycles the bolt carrier group.
The difference between the DEFCAD ‘Liberator’ and a printed AR-15 lower receiver are appreciable.
The worst-case situation I can imagine with a bad lower receiver failure on an AR-15 is the buffer tube connection severing early in the recoil cycle. Odds are the main outcome would be a failure to cycle properly with the recoil spring slowly trying to finish the job of separating the buttstock from the lower. The planets could align … badly … and the bolt might come back at you, but that seems highly unlikely.
The likelihood of a ‘Liberator’ style out-of-battery failure seem much nil since there are sturdy precision metal - principally steel - parts managing that affair.
Another situation I believe I’ve seen is the lower splitting along the X-Y plane, possibly making a yard sale of the fire control group. The firearm would be inop, but no apparent risk to the operator.
Exercise care when testing, but I’m not sure it calls for using a bench rest, string to actuate the trigger, and donning out the EOD gear for testing.
EDIT: and since I’m a bit slow, @kbraby mentioned the nil possibility of a printed lower causing an out-of-battery failure before I could finish.
