https://www.atf.gov/firearms/docs/f...nd-permit-importation-firearms-ammunition-and
See category 3...
category i—firearms
(a) Nonautomatic and semiautomatic firearms, to caliber .50 inclusive, combat shotguns, and shotguns with barrels less than 18 inches in length, and all components and parts for such firearms.
(b) Automatic firearms and all components and parts for such firearms to caliber .50 inclusive.
(c) Insurgency-counterinsurgency type firearms of other weapons having a special military application (e.g. close assault weapons systems) regardless of caliber and all components and parts for such firearms.
(d) Firearms silencers and suppressors, including flash suppressors.
(e) [Reserved]
Note: Rifles, carbines, revolvers, and pistols, to caliber .50 inclusive, combat shotguns, and shotguns with barrels less than 18 inches in length are included under Category I(a). Machineguns, submachineguns, machine pistols and fully automatic rifles to caliber .50 inclusive are included under Category I(b).
category ii—artillery projectors
(a) Guns over caliber .50, howitzers, mortars, and recoiless rifles.
(b) Military flamethrowers and projectors.
(c) Components, parts, accessories, and attachments for the articles in paragraphs (a) and (b) of this category, including but not limited to mounts and carriages for these articles.
category iii—ammunition
(a) Ammunition for the arms in Categories I and II of this section.
(b) Components, parts, accessories, and attachments for articles in paragraph (a) of this category, including but not limited to cartridge cases, powder bags, bullets, jackets, cores, shells (excluding shotgun shells), projectiles, boosters, fuzes and components therefor, primers, and other detonating devices for such ammunition.
(c)-(d) [Reserved]
Note: Cartridge and shell casings are included under Category III unless, prior to their importation, they have been rendered useless beyond the possibility of restoration for use as a cartridge or shell casing by means of heating, flame treatment, mangling, crushing, cutting, or popping.