[shortstuffsushi]

https://www.atf.gov/firearms/firearms-guides-importation-ver...

> Any type of weapon by whatever name known which will, or which may readily be converted to expel a projectile, by the action of an explosive or other propellant, the barrel or barrels of which have a bore greater than one-half inch in diameter.

Hmm, guess that would technically cover potato canons? Though the exemptions include

> a device which is neither designed nor redesigned for use as a weapon

:shrug:

[leetrout]

Only if it shoots flaming tennis balls.