Ah.... the bizzare waters of California firearms law.
Standard capacity mags can be legally owned here provided they were legally purchased prior to 1/1/2000 (burden of proof is on the prosecution)
"Rebuild kits" may be legally owned and used to rebuild existing magazines 100% - just don't end up with more magazines than you started with.
"Parts kits" may be legally owned (as long as they are never assembled in the state) taken out of state, assembled, used, disassembled and brought back in state in kit form.
or
"Parts kits" may be used to assemble legal 10 round magazines utilizing some sort of *permanent* block (permanent has never been defined by the DOJ).
You may
not import, sell, manufacture or, assemble hi cap mags. However if you "find" one, like in the desert or in the trash at a range etc... it just might be legal, the law is somewhat unclear.
You may never ever insert an 11 + round magazine into a mag locked rifle without creating an unregisterd assault weapon.
...and the fun continues.
..... Jim