27 CFR 447.41: Permit requirement.

(a) Articles on the U.S. Munitions Import List will not be imported into the United States except pursuant to a permit under this subpart. For articles subject to control under parts 478 or 479 of this chapter, a separate permit is not necessary.

(b) Articles on the U.S. Munitions Import List intended for the United States or any State or political subdivision thereof, or the District of Columbia, which are exempt from import controls of 27 CFR 478.115 shall not be imported into the United States, except by the United States or agency thereof, without first obtaining a permit under this subpart.

(c) A permit is not required for the importation of—

(1)(i) The U.S. Munitions Import List articles from Canada, except articles enumerated in Categories I, II, III, IV, VI(e), VIII(a), XVI, and XX; and

(ii) Nuclear weapons strategic delivery systems and all specifically designed components, parts, accessories, attachments, and associated equipment thereof (see Category XXI); or

(2) Minor components and parts for Category I(a) and I(b) firearms, except barrels, cylinders, receivers (frames) or complete breech mechanisms, when the total value does not exceed $100 wholesale in any single transaction.