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))
Note that it is illegal to offer or expose for sale a large-capacity magazine in California. This has the practical implication that one can not list a large-capacity magazine for sale on the internet to in or out of state buyers. One can disassemble the large-capacity magazine and sell the parts in or out of state.
Large-capacity magazines that were legally possessed by their owner in California at any time prior to January 1, 2000 can be legally re-imported into California. If you were a past resident of California that possessed large-capacity magazines in the state prior to the ban, you can move back in with them. Current residents that legally possess large-capacity magazines can travel out of the state with those large-capacity magazines and return to the state (Penal Code 12020 (b)(23)).
Repairing existing large-capacity magazines
In a letter dated November 10, 2005, Deputy Attorney General Alison Merrilees answered a list of questions about the legality of repairing and replacing parts of legally possessed large-capacity magazines.
The letter clarifies that it is legal to replace parts of a legally possessed large-capacity magazine with parts of any vintage. It also states that the possession, sale, or import of all the parts of an otherwise prohibited large-capacity magazine is not illegal though assembling them into a new large-capacity magazine would be a crime.
The general rules regarding magazine repair appear to be that as long as one ends up with the same number of legally possessed large-capacity magazines, has a plausible path of replacement parts from the original possessed large-capacity magazine to the repaired magazine, and the magazine continues to work in the firearm it was originally designed to operate in, one would not be violating the law.
There are no restrictions on modifying a legally possessed large-capacity magazine to work in another firearm as long as the magazine continues to operate in the firearm it was originally designed for.