Cal. Penal Code § 29850

(a) A violation of Section 29800, 29805, 29815, or 29820 is justifiable where all of the following conditions are met: (1) The person found the firearm or took the firearm from a person who was committing a crime against the person who found or took the firearm. (2) The person possessed the firearm no longer than was necessary to deliver or transport the firearm to a law enforcement agency for that agency’s disposition according to law or to a licensed firearms dealer for transfer or for storage pursuant to Section 29830. (3) If the firearm was transported to a law enforcement agency or to a licensed firearms dealer, it was transported in accordance with subdivision (b) of Section 25570. (4) If the firearm is being transported to a law enforcement agency or to a licensed firearms dealer, the person transporting the firearm has given prior notice to the law enforcement agency or to the licensed firearms dealer that the person is transporting the firearm to the law enforcement agency or the licensed firearms dealer for disposition according to law. (b) Upon the trial for violating Section 29800, 29805, 29815, or 29820, the trier of fact shall determine whether the defendant was acting within the provisions of the exemption created by this section. (c) The defendant has the burden of proving by a preponderance of the evidence that the defendant comes within the provisions of the exemption created by this section.