@Gepard:Typing from the scanned page linked in PCGuy's post, which is a different letter to the Spudtech one:
Department of ATF wrote:Potato cannons are generally constructed from "PVC" plastic tubing and fittings and are designed to launch a muzzle-loaded potato (or other similar size projectile) using an aerosol hair spray, propane, compressed air, etc. The propellant is ignited by mean of a barbecue grill igniter or other similar system.
Skipping to the next page, it says words to the effect of:
Things that were not designed or altered to be a weapon, are not considered firearms. Flaming (and presumably explosive) ammunition will qualify it as an illegal weapon though.
That fairly loose definition effectively covers pneumatics, hybrids or magnetic launchers.
@MrCrowley: I agree strongly. In a case like this, it would be good for Spudfiles to be vocal in the media.
I'd be willing to be an "expert" in a general Spudfiles statement if PCGuy wanted to make one.