I think others have already addressed the basic issues about owning AR-15's in California. Start with an off-list lower, and either build it without any of the so-called "evil features", or put a bullet button on it so you need to use a tool to drop your magazine.
*With apologies for linking to another firearms site* Calguns
is a great source for all your California specific questions. Here is a link to their flowchart
, which will help you navigate the complex regulations surrounding what you can and can't own with respect to so-called assault weapons.