From the California Attorney General, Department of Justice
Large Capacity Magazine
Restrictions and Exemptions (Penal Code Section 12020)
A large capacity magazine is defined as “any ammunition feeding device with a capacity to accept more than 10 rounds but shall not be construed to include a feeding device that is permanently altered so that it cannot accommodate more than 10 rounds nor shall it include any .22 caliber tube ammunition feeding device (or, effective January 1, 2002, a tubular magazine contained in a lever-action firearm).” It is important to understand that
a large capacity feeding device may be detachable or fixed, and includes any tube ammunition feeding device (other than .22 caliber or, effective January 1, 2002, a tubular magazine contained in a lever-action firearm) that
can accommodate more than 10 rounds. A large capacity magazine also includes linked ammunition with more than 10 rounds linked together or an ammunition belt with the capacity to accept more than 10 rounds.
Possession of large capacity magazines, whether by peace officers or private citizens, is not controlled.
The manufacturing, importation into the state, offering for sale, keeping for sale, exposing for sale, giving, and lending of a large capacity magazine is controlled. No person may participate in these activities without a permit issued by the Department of Justice. For exceptions, see Penal Code §§12020(b)(19)-(32).
Specified law enforcement agencies and their employees are exempt from these restrictions. These agencies and employees include any federal, state, county, city and county, or city, law enforcement agencies and employees
of those agencies while discharging their official duties, whether on-duty or off-duty, where the use is authorized by the agency within the scope of their duties. This exemption includes the sale of, giving of, lending of, importation into the state, or purchase of any large capacity magazine.
Peace officers (distinct from law enforcement agencies) who are authorized to carry firearms in the course and scope or their duties are exempted. This exemption includes the sale to, lending to, purchase of, purchase by,
receipt of, or importation into the state of large capacity magazines. For record keeping purposes, a peace officer who purchases large capacity magazines from a firearms dealer is required to provide that firearms
dealer with a copy of his or her peace officer photo identification. In the event the magazine is stamped “RESTRICTED LAW ENFORCEMENT/GOVERNMENT USE ONLY,” federal regulations require the law
enforcement officer to provide the firearms dealer with: 1) A written statement from the officer, under penalty of perjury, that the magazine is being purchased for use in performing official duties and the it is not being
acquired for personal use or for purposes of transfer or resale; and 2) a written statement from a supervisor of the purchasing officer, stating under penalty of perjury that the officer is acquiring the magazine for use in
official duties, that the magazine is suitable for use in performing official duties, and that the magazine is not being acquired for personal use or for purposes of transfer or resale.
Other allowances are made for firearms dealers; the loaning of large capacity magazines under specified conditions;
the importation into the state of previously owned magazines by residents who lawfully possessed those magazines prior to January 1, 2000 and who lawfully took them out of the state; the repair of magazines; importation of large capacity magazine by permitted individuals; the armored car industry; manufacturing large
capacity magazines for specified purposes; and prop masters (Penal Code §§ 12020(b)(21)-(32)).
Punishment – Felony or Misdemeanor. (Penal Code § 12020(a)(2))
Law Enforcement Exemption – Agencies and sworn peace officers. (Penal Code §§ 12020(b)(19), (20))
http://caag.state.ca.us/firearms/forms/pdf/awguide.pdf
Therefore YOU CAN LEGALLY posess a 20, 30, 50 or 100 round magazine in California, if you had it pre-ban, before January 1, 2000 even if you took it out of state and return.