There are two sections of the National firearms act applying to this question... See the link for the full text......
(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.
Part III - Tax on Making Firearms
Sec.
5821. Making tax.
5822. Making.
�§ 5821. Making tax
(a) Rate. -- There shall be levied, collected, and paid upon the making of a firearm a tax at the rate of $200 for each
firearm made.
(b) By whom paid. -- The tax imposed by subsection (a) of this section shall be paid by the person making the firearm.
(c) Payment. -- The tax imposed by subsection (a) of this section shall be payable by the stamp prescribed for
payment by the Secretary.
(Added Pub.L. 90-618, Title II, �§ 201, Oct. 22, 1968, 82 Stat. 1228, and amended Pub.L. 94-455, Title XIX, �§
1906(b)(13)(A), Oct. 4, 1976, 90 Stat. 1834.)
�§ 5822. Making
No person shall make a firearm unless he has (a) filed with the Secretary a written application, in duplicate, to make
and register the firearm on the form prescribed by the Secretary; (b) paid any tax payable on the making and such
payment is evidenced by the proper stamp affixed to the original application form; (c) identified the firearm to be
made in the application form in such manner as the Secretary may by regulations prescribe; (d) identified himself in
the application form in such manner as the Secretary may by regulations prescribe, except that, if such person is an
individual, the identification must include his fingerprints and his photograph; and (e) obtained the approval of the
Secretary to make and register the firearm and the application form shows such approval. Applications shall be
denied if the making or possession of the firearm would place the person making the firearm in violation of law.
(Added Pub.L. 90-618, Title II, �§ 201, Oct. 22, 1968, 82 Stat. 1228, and amended Pub.L. 94-455, Title XIX, �§
1906(b)(13)(A), Oct. 4, 1976, 90 Stat. 1834.)
ATF National Firearms Act page