As far as I understand, if you are in Ca, it is unlawful to posses a mag with capacity of more than 10 rounds.
From handgunlaw.us
Large Capacity Magazines
With limited exceptions, California law prohibits any person from manufacturing, importing into the state,
keeping for sale, offering or exposing for sale, giving, or lending any large capacity magazine. Cal. Penal
Code § 12020(a)(2), (b). A "large capacity magazine" is defined as any ammunition feeding device with the
capacity to accept more than ten rounds, but does not include any .22 caliber tube ammunition feeding
device, any feeding device that has been permanently altered so that it cannot accommodate more than ten
rounds, or any tubular magazine that is contained in a lever-action firearm. Section 12020(c)(25). California
does not ban the possession of large capacity magazines