Cal. Penal Code § 30300

(a) Any person, corporation, or dealer who does any of the following shall be punished by imprisonment in a county jail for a term not to exceed six months, or by a fine not to exceed one thousand dollars ($1,000), or by both the imprisonment and fine: (1) Sells any ammunition or reloaded ammunition to a person under 18 years of age. (2) Sells any ammunition or reloaded ammunition designed and intended for use in a handgun to a person under 21 years of age. Where ammunition or reloaded ammunition may be used in both a rifle and a handgun, it may be sold to a person who is at least 18 years of age, but less than 21 years of age, if the vendor reasonably believes that the ammunition is being acquired for use in a rifle and not a handgun. (3) Supplies, delivers, or gives possession of any ammunition to any minor who the person, corporation, or dealer knows, or using reasonable care should know, is prohibited from possessing that ammunition at that time pursuant to Chapter 1 (commencing with Section 29610) of Division 9 of Title 4 of Part 6. (b) Proof that a person, corporation, or dealer, or his or her agent or employee, demanded, was shown, and acted in reasonable reliance upon, bona fide evidence of majority and identity shall be a defense to any criminal prosecution under this section.