Cal. Penal Code - PEN § 31050. Section 31050.
(a)
Any licensed gun dealer may take possession of any assault weapon or .50 BMG rifle for the purposes of servicing or repair from any person to whom it is legally registered or who has been issued a permit to possess it pursuant to this chapter.
(b)
Any licensed gun dealer may transfer possession of any assault weapon or .50 BMG rifle received pursuant to subdivision (a), to a gunsmith for purposes of accomplishing service or repair of that weapon. A transfer is permissible only to the following persons:
(1)
A gunsmith who is in the dealer’s employ.
(2)
A gunsmith
with whom the dealer has contracted for gunsmithing services.
(c)
In order for paragraph (2) of subdivision (b) to apply, the gunsmith receiving the assault weapon or .50 BMG rifle shall hold all of the following:
(1)
A dealer’s license issued pursuant to Chapter 44 (commencing with Section 921) of Title 18 of the United States Code and the regulations issued pursuant thereto.
(2)
Any business license required by a state or local governmental entity.