26 U.S. Code § 5845
(a)
Firearm
The term “firearm” means
921 of title
18, United States Code); and (a)
Firearm
(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) any other weapon, as defined in subsection (e);
921 of title
18, United States Code); and
(6) a machinegun;
921 of title
18, United States Code); and
(7) any silencer (as defined in section
921 of title
18, United States Code); and
(8) a destructive device. The term “firearm” shall not include an antique firearm or any device (other than a machinegun or destructive device) which, although designed as a weapon, the Secretary finds 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.