(1) A shotgun having a barrel or barrels of less than 18 inches in length.
(2) A weapon made from a shotgun, if such weapon as modified has an overall length of less than 26 inches or a barrel or barrels of less than 18 inches in length.
(3) A rifle having a barrel or barrels of less than 16 inches in length.
(4) A weapon made from a rifle if such weapon as modified has an overall length of less than 26 inches or a barrel or barrels of less than 16 inches in length.
(5) A machine gun.
(6) A muffler or a silencer for any firearm whether or not such firearm is included within this definition. The term shall not include an antique firearm or any device (other than a machine gun) which, although designed as a weapon, by reason of the date of its manufacture, value, design, and other characteristics is primarily a collector's item and is not likely to be used as a weapon. For purpose of this definition, the length of the barrel on a shotgun or rifle shall be determined by measuring the distance between the muzzle and the face of the bolt, breech, or breechlock when closed and when the shotgun or rifle is cocked. The overall length of a weapon made from a shotgun or rifle is the distance between the extreme ends of the weapon measured along a line parallel to the center line of the bore.
32 CFR § 552.126
For the purpose of this part, the following definitions apply: