Source: https://www.atf.gov/firearms/firearms-guides-importation-verification-firearms-ammunition-and-implements-war-import
Timestamp: 2019-04-23 06:32:05+00:00

Document:
It shall be unlawful for any person other than an FFL, knowingly to import, or bring into the United States, any firearms or ammunition. 18 U.S.C. § 922(a)(1).
However, as provided in 18 U.S.C. § 925, the GCA generally allows the importation of sporting firearms and ammunition and certain surplus military firearms classified as curios or relics.
To qualify for importation under 18 U.S.C. § 925(d)(3), a firearm or ammunition must not fall under the definition of firearm as defined in 26 U.S.C. § 5845(b), and must be of a type generally recognized as particularly suitable for or readily adaptable to sporting purposes.
Firearms such as single shot, lever action, bolt action and certain semiautomatic long guns with generally recognized sporting features.
Note: ATF has determined that certain features designed for military application are indicative of non-sporting rifles and shotguns. Features which are not recognized as sporting include, but are not limited to, folding or telescoping stocks, pistol grips that protrude conspicuously beneath the action of the weapon, a bayonet or bayonet mount, a flash suppressor or threaded barrel designed to accommodate a flash suppressor, a grenade launcher and night sights. These features as well as other information concerning a particular firearm may result in ATF classifying a rifle or shotgun as non-sporting. Additional information regarding this subject is available in the Report and Recommendation of the ATF Working Group on the Importability of Certain Semiautomatic Rifles or The Department of Treasury Study on the Suitability of Modified Semiautomatic Assault Rifles.
Sporting ammunition is all ammunition except, tracer or incendiary rounds, ammunition for destructive devices, less than lethal (i.e., rubber projectiles) and armor piercing ammunition as defined in 18 U.S.C. § 921(a)(17) and 27 CFR § 478.11.
A surplus military firearm is any firearm which has ever been possessed by a regular or irregular military force. Surplus military firearms are prohibited from importation under 18 U.S.C. § 925(d)(3); however, § 925(e) authorizes licensed importers (FFL type 08 or 11) to import surplus military rifles and shotguns classified as curios or relics; and handguns classified as curios or relics which meet the sporting criteria. In order to qualify for importation the firearms must be in their original military configuration and cannot have been sporterized. Further, under the AECA, the importation of U.S.-origin surplus military firearms is generally prohibited without retransfer authorization from the Department of State. 27 CFR § 447.57.

References: § 922
 § 925
 § 925
 § 5845
 § 921
 § 478
 § 925
 § 925
 § 447