This was quickly built as an experiment to see if it would be worth going ahead and building a fancy version. It's a rubberband-powered dart gun, and the cool thing about it is the more rubberbands you use, the more powerful it becomes.
Rubberbands hook onto dowels at the end of the barrel and stretch around a pin on the back of the ramrod. It works best if the same amount of rubberbands go on each side. The trigger is a rooster catch used to hold cabinet doors shut. When the trigger is released the ramrod slams whatever is inside the barrel out with pretty hefty force. I put a 16-penny nail in it and shot it into a piece of plywood, and it stuck! Any pencil-sized object works as ammunition.
One downside to this design: You can't aim down at something or your ammunition will fall out of the barrel. BUT anything horizontal or upwards better look out!
At first I thought this would be a fun kid's toy. Now that it's finished though, I can see how kids shooting nails could end badly. CLICK TO ENLARGE