A rifle is defined, 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 cartridge to fire only a single projectile through a rifled bore for each single pull of the trigger.
A rifle is subject to the NFA only if the rifle has a barrel or barrels of less than 16 inches in length. A weapon made from a rifle 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 16 inches in length.
[26 U.S.C. §§ 5845(a)(3)-(4), (c)]