As far as weapons other than slingshots go, I'm a big fan of potato cannons. In case you don't know what they are, they're assemblies of PVC pipe with a sparker and a chamber. Hairspray is sprayed into the chamber, and the sparker is pressed. This sends a potato hurling out of the barrel. The cannons that I make, however. Use compressed air and a valve. I'm interested in any stories of someone else's cannon, if they have one.