Transferring firearms Interstate
Interstate transfer is governed by a combination of Federal and State laws. 18 USC 922 (a) makes it unlawful
(2) for any importer, manufacturer, dealer, or collector licensed under the provisions of this chapter to ship or transport in interstate or foreign commerce any firearm to any person other than a licensed importer, licensed manufacturer, licensed dealer, or licensed collector, ... (3) for any person, other than a licensed importer, licensed manufacturer, licensed dealer, or licensed collector to transport into or receive in the State where he resides (or if the person is a corporation or other business entity, the State where it maintains a place of business) any firearm purchased or otherwise obtained by such person outside that State, except that this paragraph (A) shall not preclude any person who lawfully acquires a firearm by bequest or intestate succession in a State other than his State of residence from transporting the firearm into or receiving it in that State, if it is lawful for such person to purchase or possess such firearm in that State, (B) shall not apply to the transportation or receipt of a firearm obtained in conformity with subsection (b)(3) of this section, and (C) shall not apply to the transportation of any firearm acquired in any State prior to the effective date of this chapter; (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; except that this paragraph shall not apply to (A) the transfer, transportation, or delivery of a firearm made to carry out a bequest of a firearm to, or an acquisition by intestate succession of a firearm by, a person who is permitted to acquire or possess a firearm under the laws of the State of his residence, and (B) the loan or rental of a firearm to any person for temporary use for lawful sporting purposes;
Illegal for the DEALER
(a)(2) tells us a licensed dealer may ship interstate only to another licensed dealer. There is an exception for "returning a firearm or replacement firearm of the same kind and type to a person from whom it was received".
Illegal for the private BUYER
(a)(3) tells us that unlicensed individuals may not go to a state where they do not live, buy a gun, take delivery of the gun, and bring it back to the state of residence.
Illegal for the private SELLER
(a)(5) tells us that unlicensed individuals may not sell a gun to unlicensed individuals who do not live in the same state as the seller.
Penalties for violation of (a)(2), (a)(3), and (a)(5) are the 'default' provisions of 18 USC 924 (a)(1)(D):
... whoever— ... (D) willfully violates any other provision of this chapter, shall be fined under this title, imprisoned not more than five years, or both.
Use an FFL
The effect of these: interstate transfers must go through a Federal Firearms Licensee (FFL) in the buyer's state of residence.
The Bureau of Alcohol, Firearms, Tobacco and Explosives (variously BATFE, BATF, ATF) says in its FAQ
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.
Federal Exceptions, California complications
As noted above, return of a firearm from a dealer or manufacturer to the individual who sent the firearm is an exception.
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.
California residents are subject to Penal Code 12070
12070. (a) No person shall sell, lease, or transfer firearms unless he or she has been issued a license pursuant to Section 12071.
(d) Where neither party to the transaction holds a dealer's license issued pursuant to Section 12071, the parties to the transaction shall complete the sale, loan, or transfer of that firearm through a licensed firearms dealer pursuant to Section 12082.
Penal Code 12071 is the California firearms dealer licensing requirement. Complying "with State laws" requires using a properly licensed California FFL.
The CA-DOJ FAQ is clear:
All firearms purchases and transfers, including private party transactions and sales at gun shows, must be made through a licensed dealer under the Dealer Record of Sale (DROS) process.
Interstate inheritance from anyone is exempt from (a)(3) -- California allows only immediate family members - grandparents, parents, children, grandchildren. Penal Code 12078(c)(1) and (c)(2)
Note that Penal Code 12070(b) has several exceptions to the California FFL transfer requirement, including language as "Sales, deliveries, or transfers of firearms by persons who reside outside this state and are licensed outside this state pursuant to Chapter 44 ... of Title 18 of the United States Code" and none of those exempt unlicensed individuals. The Legislature knows how to make exceptions, and has not made any for unlicensed individuals.
Handguns brought or shipped interstate into California for sale must be on the Roster.
Private Party Transfers
Firearms brought or shipped interstate into California for sale cannot be transferred in a California Private Party Transfer.
Curios and Relics
Holders of a Federal Curio and Relic License ("Collectors" license) have further exemptions; see that article.