I never brought any firearms back myself but knew other crewmembers who did, no problem at all with the proper paperwork.
Non-Licensed Resident of the U.S.
Any person residing in the U.S. who does not hold an FFL. A non-licensed resident of the U.S may obtain an approved ATF F 6 (Part I) import permit to import sporting ammunition for his or her own use but not for resale or other commercial purpose. However, a non-licensed resident of the U.S. may not obtain an approved ATF Form 6 import permit to import nonsporting ammunition or any firearm. Any non-licensee who desires to import a firearm into the U.S. must have the firearm imported on his or her behalf by a qualified FFL, and the subsequent transfer of the firearm to the non-licensee must be in compliance with Federal, State and local law.