26 USC 4181 - Imposition of tax

There is hereby imposed upon the sale by the manufacturer, producer, or importer of the following articles a tax equivalent to the specified percent of the price for which so sold: Pistols. Revolvers. Firearms (other than pistols and revolvers). Shells, and cartridges.

26 USC 4182 - Exemptions

(a) Machine guns and short barrelled firearms 
The tax imposed by section 4181 shall not apply to any firearm on which the tax provided by section 5811 has been paid.
(b) Sales to defense department 
No firearms, pistols, revolvers, shells, and cartridges purchased with funds appropriated for the military department shall be subject to any tax imposed on the sale or transfer of such articles.
(c) Small manufacturers, etc. 

(1) In general 
The tax imposed by section 4181 shall not apply to any pistol, revolver, or firearm described in such section if manufactured, produced, or imported by a person who manufactures, produces, and imports less than an aggregate of 50 of such articles during the calendar year.
(2) Controlled groups 
All persons treated as a single employer for purposes of subsection (a) or (b) of section 52 shall be treated as one person for purposes of paragraph (1).
(d) Records 
Notwithstanding the provisions of sections 922 (b)(5) and 923 (g) of title 18, United States Code, no person holding a Federal license under chapter 44 of title 18, United States Code, shall be required to record the name, address, or other information about the purchaser of shotgun ammunition, ammunition suitable for use only in rifles generally available in commerce, or component parts for the aforesaid types of ammunition.