Defense Distributed is a project that has been working on 3D printer files that allow users to make their own guns, bit by bit.
Sadly, or fortunately, depending on one's view, the completed weapon failed to hold up in tests.
It only took six shots for the gun to fail, even though the ammunition used was specifically designed for lower recoil.
HaveBlue was the creator of the blueprint for the AR lower receiver of the weapon (the part that receives the rifle cartridge from the magazine).
According to NBC News and WikiWep DevBlog, the firearm basically broke in half. Fortunately, no one was hurt, even though the danger remains. The spring and buffer just popped out of the tube and fell uselessly to the ground.
For a more encouraging example of 3D printing, see this autobot.