You have to buy or receive (from an FFL) handguns in your own state as it is illegal to buy a handgun in a state in which you are not a resident.
Longarms are not so encumbered - it is legal to purchase a longarm from any state that borders your own.
Er, correction - it is legal to purchase a longarm in any state, period, under Federal law. State law may restrict it to “neighboring states” or “not at all”.
From BATF:
B. UNLICENSED PERSONS
(B1) To whom may an unlicensed person transfer firearms under the GCA?
A person may sell a firearm to an unlicensed resident of his or her state, if the buyer is not prohibited by law from receiving or possessing a firearm, or to a licensee in any state. 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( b)( 3), 27 CFR 178.29]
(B2) From whom may an unlicensed person acquire a firearm under the GCA?
A person may only buy a firearm within the person’s own state, except that he or she may buy 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. [18 U. S. C 922( a)( 3) and (5), 922( b)( 3), 27 CFR 178.29]
(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?
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 (5), 922( b)( 3), 27 CFR 178.29]