About 30 miniutes ago at my local community center they had a party where about 50 people at a time in a gymnasium (I was one!!!
) had simple puematic cannons that shot ping pong balls randomly found on the ground. 
The basic idea is a flexable bottle (that you can hit and will just flex) is attached to a foot long cardboard tube the ID of a ping pong ball.
You put the ping pong ball in the barrel. Put the gun against your waist, and hit the chamber expelling the ball out at rspectible speeds (enough to leave a big welt)
There was speedball type cover everywhere and total darkness but a flashing light that made you felt you were having a siezure or haveing a nightmare.
It was really fun, the lights made my friend look like a orange team guy and I shot him from about 4 feet and gave him a big welt in his stomach.
I only got hit by 2 kids in the back, but chased one around hiting him 3 times in the back and thigh.
Of course someone is going to say "NEVER SHOOT PROJECTILES AT HUMANS" But have you ever played paintball or airsoft?
With a good arm you leave huge welts at close range, but we all wore goggles. I got bettween the eyes once, but i was wearing goggles. 8)
