Technically it is illegal to bring a magazine with a capacity greater than 10 rounds into CA. If that concerns you, just disassemble the mag. (It is not illegal to import magazine parts into CA, but importing a high cap mag is illegal) -- it's CA, don't try to rationalize it. Transport the firearm unloaded and locked and any ammunition separately from the firearm with both out of immediate reach. That is the only way to be perfectly legal in CA.