Last year, his group famously demonstrated that it could use a 3D-printed “lower” for an AR-15 semi-automatic rifle—but the gun failed after six rounds. Now, after some re-tooling, Defense Distributed has shown that it has fixed the design flaws and a gun using its lower can seemingly fire for quite a while. (The AR-15 is the civilian version of the military M16 rifle.)
The lower, or "lower receiver" part of a firearm, is the crucial part that contains all of the gun's operating parts, including the trigger group and the magazine port. (Under American law, the lower is what's defined as the firearm itself.) The AR is designed to be modular, meaning it can receive different types of “uppers” (barrels) as well as different-sized magazines.