Earlier this year, amateur gunsmiths got together to see if they could print out some parts that could be used to construct a fully functional AR-15 semi-automatic rifle. Last weekend, a 3D-printed lower receiver was tested to failure shooting real bullets, and made it through six shots before suffering what you could legitimately call a catastrophic structural failure.
Note that this is a test of a 3D-printed lower receiver only; all of the other parts on the gun are actual gun parts, most of which are made of metal and designed to handle with the 50,000 psi stresses that come with firing a bullet. The receiver, however, doesn't have to deal with any of that, which is why these guys are testing the part to failure themselves and not clamping it to a bench instead:
Due to the fact that all the dangerous stresses are contained in the bolt and barrel extension — in the event of a lower failure, the only damage the operator faces is to the ego. The spring and buffer simply popped out the the tube and fell to the ground. The buffer detent and spring very lightly jammed the bolt halfway between open and in battery. It was easily cleared and no damage was sustained by the detent, spring, or bolt.
The hope was that they'd get 20 shots out of the gun, but they only managed six. Here's the video:
And here's a close-up of the failure point:
The great thing about 3D printing, though, is that these guys can now go back and say, "okay, how to we modify our design to reinforce the area that failed to be able to handle more stress?" They've already got ideas, like adding thickness or additional supporting material, and it's relatively easy to just print out a second iteration to evaluate. It's like taking a complex industrial design process and making it accessible (nearly) to the individual consumer. People may not approve of these techniques being used to build a better gun, but this is the future of making all sorts of stuff, and there's no looking back.