(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—
(1)
is being imported or brought in for scientific or research purposes, or is for use in connection with competition or training pursuant to chapter
401 of title
10;
(2)
is an unserviceable firearm, other than a machinegun as defined in section 5845(b) of the Internal Revenue Code of 1986 (not readily restorable to firing condition), imported or brought in as a curio or museum piece;
(3)
is of a type that does not fall within the definition of a firearm as defined in section 5845(a) 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; or
(4)
was previously taken out of the United States or a possession by the person who is bringing in the firearm or ammunition.
The Attorney General shall permit the conditional importation or bringing in of a firearm or ammunition for examination and testing in connection with the making of a determination as to whether the importation or bringing in of such firearm or ammunition will be allowed under this subsection.