Generally, permanent imports of defense articles enumerated on the U.S. Munitions Import List (USMIL) at 27 CFR 447.21 require an ATF Form 6, Application and Permit for Importation of Firearms, Ammunition and Implements of War (5330.3A).
Persons in the United States engaged in the business of importing defense articles enumerated on the USMIL must register by making an application on the ATF Form 4587, Application to Register as an Importer of U.S. Munitions Import List Articles (5330.4).
[22 U.S.C. § 2778; 27 CFR § 478.111-478.120; 27 CFR § 447.31-447.35, 27 CFR § 447.41-447.45]