Large-capacity magazine restrictions
As part of expanding the definition of Assault Weapons with the passage of SB-23 in July 1999, prohibitions on magazine capacity sizes were introduced into the Penal Code. SB-23 added a new term, "large-capacity magazine."
Penal Code 12020 (a)(2) states:
12020 (a)(2) Commencing January 1, 2000, manufactures or causes to be manufactured, imports into the state, keeps for sale, or offers or exposes for sale, or who gives, or lends, any large-capacity magazine.
The California Department of Justice Bureau of Firearms FAQ summarizes the restrictions well:
9. If I have a large-capacity magazine, do I need to get rid of it? No. Continued possession of large-capacity magazines (able to accept more than 10 rounds) that you owned in California before January 1, 2000, is not prohibited. However as of January 1, 2000, it is illegal to buy, manufacture, import, keep for sale, expose for sale, give or lend any large-capacity magazine in California except by law enforcement agencies, California peace officers, or licensed dealers. (PC Section 12020 (b)(19-29))