The NFA defines shotgun, in part, as a weapon designed or redesigned, made or remade, and intended to be fired from the shoulder and designed or redesigned and made or remade to use the energy of the explosive in a fixed shotgun shell to fire through a smooth bore either a number of projectiles (ball shot) or a single projectile for each pull of the trigger.
A shotgun is a firearm subject to the NFA if the shotgun has a barrel or barrels of less than 18 inches in length. A weapon made from a shotgun is also a firearm subject to the NFA if the weapon as modified has an overall length of less than 26 inches or a barrel or barrels of less than 18 inches in length.
[26 U.S.C. §§ 5845(a)(1)-(2), (d)]