Straight From the Horse's Mouth: [BATFE Website]
http://www.atf.gov/firearms/faq/licensees-conduct-of-business.html#out-of-state-resident
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Q: May a licensed dealer sell a firearm to a non-licensee who is a resident of another State?
Generally, a firearm may not lawfully be sold by a licensed dealer to a non-licensee who resides in a State other than the State in which the seller’s licensed premises is located. However, the sale may be made if the firearm is shipped to a licensed dealer whose business is in the purchaser’s State of residence and the purchaser takes delivery of the firearm from the dealer in his or her State of residence.
In addition, a licensee may sell a rifle or shotgun to a person who is not a resident of the State where the licensee’s business premises is located in an over-the-counter transaction, provided the transaction complies with State law in the State where the licensee is located and in the State where the purchaser resides.
[18 U.S.C. 922(b)(3)]
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The above refers to transfers involving licensees. Here is more information about transactions between non-licensees:
Q: To whom may an unlicensed person transfer firearms under the GCA?
A person may sell a firearm to an unlicensed resident of his State, if he does not know or have reasonable cause to believe the person is prohibited from receiving or possessing firearms under Federal law. A person may loan or rent a firearm to a resident of any State for temporary use for lawful sporting purposes, if he does not know or have reasonable cause to believe the person is prohibited from receiving or possessing firearms under Federal law. A person may sell or transfer a firearm to a licensee in any State. However, a firearm other than a curio or relic may not be transferred interstate to a licensed collector.
[18 U.S.C. 922(a)(3) and (5), 922(d), 27 CFR 478.29 and 478.30]
Q: From whom may an unlicensed person acquire a firearm under the GCA?
A person may only acquire a firearm within the person’s own State, except that he or she may purchase or otherwise acquire a rifle or shotgun, in person, at a licensee’s premises in any State, provided the sale complies with State laws applicable in the State of sale and the State where the purchaser resides. A person may borrow or rent a firearm in any State for temporary use for lawful sporting purposes.
[18 U.S.C. 922(a)(3) and (5), 922(b)(3), 27 CFR 478.29 and 478.30]
Q: May an unlicensed person obtain a firearm from an out-of-State source if the person arranges to obtain the firearm through a licensed dealer in the purchaser’s own State?
A person not licensed under the GCA and not prohibited from acquiring firearms may purchase a firearm from an out-of-State source and obtain the firearm if an arrangement is made with a licensed dealer in the purchaser’s State of residence for the purchaser to obtain the firearm from the dealer.
[18 U.S.C. 922(a)(3) and 922(b)(3)]