This is taken from the California Attorney General Assault Weapons Guide
Note that this is from 2001.
About the 20 round mag, basically:
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.
^^^Now I don't get this part, does that mean that it's legal to have one or not?!^^^
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.
^^^Why does this **** have to be so confusing?^^^
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.