Engineering student James Patrick has built a working 3D-printable revolver that uses a rubber band and a nail to fire .22 bullets.
The revolver can hold six or eight bullets and moves the cylinder from position to position after each shot. Patrick has built in a protective system that keeps the gun from firing when dropped. His last project, called the Songbird, a single shot model, is also available on his site.
These sorts of guns are at once interesting and damaging to the world of 3D printing hobbyists. While building a gun is an excellent proof-of-concept for heavy-duty 3D printed hardware, it also rings plenty of bells on the regulatory front.
You can download the plans here and the system requires a certain amount of metal so it will be detectable by metal detectors. It must also be printed on a heavy-duty ABS printer so it will not explode in the hand. You can watch Patrick fire the gun below.