18 USC 925:
(d) The Attorney General shall authorize a firearm or ammunition to be imported or brought into the United States or any possession thereof if the firearm or ammunition—
[irrelevant sections cut]
(3) is of a type that does not fall within the definition of a firearm as defined in section 5845(a) [ed. 5845 defines "machine guns"] of the Internal Revenue Code of 1986 and is generally recognized as particularly suitable for or readily adaptable to sporting purposes, excluding surplus military firearms, except in any case where the Attorney General has not authorized the importation of the firearm pursuant to this paragraph, it shall be unlawful to import any frame, receiver, or barrel of such firearm which would be prohibited if assembled;...
[more irrelevant verbiage cut]
(e) Notwithstanding any other provision of this title, the Attorney General shall authorize the importation of, by any licensed importer, the following:
(1) All rifles and shotguns listed as curios or relics by the Attorney General pursuant to section 921 (a)(13), and...