27 CFR § 447.57 - U.S. military defense articles.
(1) Notwithstanding any other provision of this part or of parts 478 or 479 of this chapter, no military defense article of United States manufacture may be imported into the United States if such article was furnished to a foreign government under a foreign assistance or foreign military sales program of the United States.
(2) The restrictions in paragraph (a)(1) of this section cover defense articles which are advanced in value or improved in condition in a foreign country, but do not include those which have been substantially transformed as to become, in effect, articles of foreign manufacture.
(b) Paragraph (a) of this section will not apply if:
(1) The applicant submits with the ATF Form 6 - Part I application written authorization from the Department of State to import the defense article; and
(2) In the case of firearms, such firearms are curios or relics under 18 U.S.C. 925(e) and the person seeking to import such firearms provides a certification of a foreign government that the firearms were furnished to such government under a foreign assistance or foreign military sales program of the United States and that the firearms are owned by such foreign government. (See § 478.118 of this chapter providing for the importation of certain curio or relic handguns, rifles and shotguns.)
(c) For the purpose of this section, the term “military defense article” includes all defense articles furnished to foreign governments under a foreign assistance or foreign military sales program of the United States as set forth in paragraph (a) of this section.