Seems it’s been a while since we’ve done a decent write up. The new AR lower was SLA and FDM printed. The FDM printer was a used Dimension SST, and the material was p400 ABS.
This lower was built out horizontally, with improved cross sectional areas at the rear take down pins, and more curved and filled spaces below the buffer tower and inside the rear of the receiver. The idea was to take out as many angles and stress risers as possible in the rear, build out critical thicknesses for strength, and let the piece act more as a spring.
These changes had been in note form for close to two months, but, as you may be aware, we had been directing a significant part of our efforts at magazines in that time. And yes, AK magazines are being tested again this weekend.
Importantly, we used a 1/8” compression bushing in the back of the piece for a rear take down pin, but a 1/8” shoulder bolt would work too. These smaller holes were touched up with a drill bit.
This is the first publicly printed AR lower demonstrated to withstand a large volume of .223 without structural degradation or failure. The actual count was 660+ on day 1 with the SLA lower. The test ended when we ran out of ammunition, but this lower could easily withstand 1,000 rounds.
Day 2, we broke out the FDM lower where, after 88 rounds, the rifle experienced an unrelated mechanical failure (the bolt had a worn cam pin that prevented the bolt from opening all the way). Army protocol says beat the buttstock on the ground, but after some effort this only made a split at the top of the buffer tower, ending the test.
But the big takeaway is that you can do what we did on a printer like the Ultimaker or RepRap. The pieces can be made cheaply, and the file is free at DEFCAD.
And about DEFCAD. Amazing things are happening there. My personal apologies for the times the site has been down. We were unprepared for the volume of visitors and downloads, and have also been hopping VPS’s.
I will include a time lapse of one of these prints soon.
Finally, I want to address the communications and requests we get asking us to explain our printing setup or make recommendations: We are working on releasing a primer. We understand the need to get simple tutorials out for the beginning gun printer. Recommendations are more difficult, since we’ve yet to experiment with more than a few materials across a mere 5 machines. We’re still learning as well, but that’s no excuse for not sharing.
Wither Gun Control.