What dstorm1911 said.
922. Unlawful acts
(a) It shall be unlawful—
(5) for any person (other than a licensed importer, licensed manufacturer, licensed dealer, or licensed collector) to transfer, sell, trade, give, transport, or deliver any firearm to any person (other than a licensed importer, licensed manufacturer, licensed dealer, or licensed collector) who the transferor knows or has reasonable cause to believe does not reside in (or if the person is a corporation or other business entity, does not maintain a place of business in) the State in which the transferor resides;
It continues with exemptions for inherited firearms and loaning a firearm for temporary sporting use.