I made this pneumatic rifle when I was a school, although the barrel is smooth, so I suppose it can not be called a rifle.
It is made from scrap wood, old pipes, a small valve, a spring and an air pump.
To operate it, you push a 22 air gun pellet down the barrel with the piece of car aerial, then close the valve before pumping some air into the air reservoir. When the trigger is pulled the spring opens the valve and the trapped compressed air forces the pellet out of the barrel - knocking the tin can flying! I also made the shooting gallery