When I was about twelve years old, my friend and I wanted to construct are own homemade paintball guns.  This small pneumatic rifle was the result. The inexpensive launcher actually worked quite well.

Theory: By pumping air into the tank and letting all the air out at once it produces enough pressure to shoot the paintball.  This small cannon uses a supersoaker tank as a chamber.  The hand pump attached is used to pump up the chamber.

How to fire: Use the small hand pump on the side of the "air chamber" and pump it about 10-15 times. Muzzle load the ammunition (paintball, mothball, wooden ball, etc.), flip the "ball valve" and it will fire the ammunition at a very high speed.

The construction is pretty straight forward but if you would like step-by-step, easy to follow plans you can order them here.