This project consists of retrofitting a vacuum cleaner to fire a projectile through the usage of air pressure (or lack thereof). The parts: Vacuum cleaner, obtained from a garage sale, 12″ wooden dowel, 10 feet PVC pipe, and Tee fitting for the pipe. I’ve seen bigger versions of these do some amazing things like send a ping pong ball through metal cans. Link.
$5 Vacuum Bazooka