27 CFR 478.33: Stolen firearms and ammunition.

No person shall transport or ship in interstate or foreign commerce any stolen firearm or stolen ammunition knowing or having reasonable cause to believe that the firearm or ammunition was stolen, and no person shall receive, possess, conceal, store, barter, sell, or dispose of any stolen firearm or stolen ammunition, or pledge or accept as security for a loan any stolen firearm or stolen ammunition, which is moving as, which is a part of, which constitutes, or which has been shipped or transported in, interstate or foreign commerce, either before or after it was stolen, knowing or having reasonable cause to believe that the firearm or ammunition was stolen.