If you want to build a projectile weapon system but never want to run out of ammo, try this how-to and build a giant cannon that shoots air balls! With easily available parts, costing under $15, you'll be ready to shoot in no time. Add fog and you have yourself a giant smoke ring shooter. With practice you can shoot hats off people with a harmless ball of air.
You'll need: a circular trash can, 2 48" bungee cords, duct tape, and some plastic sheeting and a golf ball.