It depends on if it's a rifle/shotgun or handgun. This is from ATF:
(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]
It appears you don't need to do anything special in terms of rifles/shotguns. For handguns, you need to have an FFL in CA handle the transfer for you.
This may not apply to you, but if you are moving to CA from out-of-state and plan to brings handguns with you, you are considered a "personal handgun importer" and must report all handguns acquired outside CA to the Department of Justice within 60 days.
Looking at B2, I don't know if that means the out-of-state dealer would have to impose a 10-day waiting period on you, as that is CA law for all firearms. It's probably up to him.
Anyway, looking at the Federal law, it appears you can buy CA legal rifles/shotguns in person out-of-state and bring them back in and not have to report anything. I could be wrong, so you might want to speak to a CA FFL. There are some hiding on this board, I'm sure.