blackhawk556 covered it.
You cannot bring the assembled "high capacity" magazines with you.
You need to block them at 10 rounds.
Bringing in standard capacity magazines is a serious crime.
When you arrive in California you need to fill out the form and pay money for the privilege of registering any handguns you have. The fee is per handgun.
These handguns then go into a database that LEO can see, and that is cross checked for prohibited persons automatically by software.
So if at any time you become prohibited, from say one of the numerous prohibiting misdemeanors, a felony, or other things in the future it will automatically red flag you as a bad guy known to have a gun. Armed and dangerous bad guy.
For an example take this current thread of a guy that become a prohibited person without committing a crime and was then red flagged by the system:
http://www.thehighroad.org/showthread.php?t=598421
Additionally you cannot have your pistol barrel threaded. A semi-automatic pistol with a threaded barrel is an illegal 'assault weapon' in California, a felony subjecting you to up to 10 years in prison.
You are free to bring and register the gun with California, and then proceed to only use 10 round magazines with it, wasting most of the magazine space and having a larger grip than necessary for nothing.