Sorry for the triple post, but I stopped being lazy:
(B2) From whom may an unlicensed person acquire a firearm under the GCA? [Back]
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]
(B3) 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? [Back]
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)]
Note also that 18 U. S. C. (a)(5) makes it unlawful for a resident of a state to transfer or deliver a gun to someone he should know is not also a resident of that state.
Here is the law. Even private party sales are out of the question for handguns across state lines.