IANAL, but I have read the ATF books
Under US Federal law, potato cannons are not firearms.
Some states have particularly irritating regs - MA used to regulate them as firearms, because MA regulates EVERYTHING! (Posession of a single BB could land you a year in jail. No kidding)
This changed in 1998, now legal in MA.
However, all this is moot as long as you don't get caught. Further, although technically illegal, most cops really won't care unless you've just used it to shoot some little old lady crossing the street and forcing her under the wheels of a pulp truck.
Just my take
-Ben