26 U.S.C. § 5844

No firearm shall be imported or brought into the United States or any territory under its control or jurisdiction unless the importer establishes, under regulations as may be prescribed by the Secretary, that the firearm to be imported or brought in is— (1) being imported or brought in for the use of the United States or any department, independent establishment, or agency thereof or any State or possession or any political subdivision thereof; or (2) being imported or brought in for scientific or research purposes; or (3) being imported or brought in solely for testing or use as a model by a registered manufacturer or solely for use as a sample by a registered importer or registered dealer; except that, the Secretary may permit the conditional importation or bringing in of a firearm for examination and testing in connection with classifying the firearm.