I started a thread about the legality of potato guns years ago.
http://www.theoutdoorstrader.com/threads/278508-Legality-of-the-Potato-Gun?highlight=potato+gun
They are completely legal as long as you don't shoot anything with them causing damage to the object. Basically if you shoot it off just for distance you are cool. However, if you shoot a target with it, it becomes a destructive device. I've done research at this point and had a brief discussion with ATF and Sheriff about it. Both also said they they had 0 knowledge of anyone ever being charged for having a regular potato gun. The ATF lady had an interesting story about someone shooting 1 liter bottles full of gas out of one and effectively making molotov RPG thingy. They caught him, guess where, Youtube...
Sounds like the running with scissors Postal 2 game napalm launcher