From the GCA as codified in 18 USC §921(a)(3)(C):
(3) The term “firearm” means (A) any weapon (including a starter gun) which will or is designed to or may readily be converted to expel a projectile by the action of an explosive;
(B) the frame or receiver of any such weapon;
(C) any firearm muffler or firearm silencer; or
(D) any destructive device. Such term does not include an antique firearm.
From the NFA as codified in 26 USC §5845(a)(7), note the reference to the GCA in 18 USC §921.
(a) Firearm The term “firearm” means
(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);
(6) a machinegun;
(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.
18 USC §921(a)(24)
(24) The terms “firearm silencer” and “firearm muffler” mean any device for silencing, muffling, or diminishing the report of a portable firearm, including any combination of parts, designed or redesigned, and intended for use in assembling or fabricating a firearm silencer or firearm muffler, and any part intended only for use in such assembly or fabrication.
27 CFR §478.124
(a) A licensed importer, licensed manufacturer, or licensed dealer shall not sell or otherwise dispose, temporarily or permanently, of any firearm to any person, other than another licensee, unless the licensee records the transaction on a firearms transaction record, Form 4473: Provided, That a firearms transaction record, Form 4473, shall not be required to record the disposition made of a firearm delivered to a licensee for the sole purpose of repair or customizing when such firearm or a replacement firearm is returned to the person from whom received.