I remember when I was a kid, I read about a potato gun in a Johnson Smith catalog. (They no longer sell potato guns or many of the weird things that appealed to me when I was ten.)
Here's one you can build yourself. Or if you're not handy, just plunk down a couple of bucks -- okay, from $60 to $2500 -- and you can have one built.
BTW, the Bureau of Alcohol, Tobacco, and Firearms said the potato gun is perfectly legal. [Thanks, Harold M.]
