Make:

Alexei writes - "One day I was making a simple little elastic gun to shoot Lego blocks and I thought to myself, "it would be cooler if it had some kind of trigger." The next thing I knew, I was involved in a competition with my own self - I tried to imagine the most impossible thing to make Lego do, and then set out to figure out a way to do it. About 300 hours later (in hindsight I probably should have spent the time canvassing for UNICEF or something more noble) I had a semi-automatic pump-action lego crossbow that took four blocks in the handle and one in the chamber. It can fire a standard lego brick about 20 yards. It's not terribly accurate, but believe me, you don't want to fire this thing at the family pet." - [via] Link.