California Code, Penal Code - PEN § 25250

(a) Commencing July 1, 2017, every person shall report the loss or theft of a firearm he or she owns or possesses to a local law enforcement agency in the jurisdiction in which the theft or loss occurred within five days of the time he or she knew or reasonably should have known that the firearm had been stolen or lost.

(b) Every person who has reported a firearm lost or stolen under subdivision (a) shall notify the local law enforcement agency in the jurisdiction in which the theft or loss occurred within five days if the firearm is subsequently recovered by the person.

(c) Notwithstanding subdivision (a), a person shall not be required to report the loss or theft of a firearm that is an antique firearm within the meaning of subdivision (c) of Section 16170.