The AR 15 is legal in California, as a matter of fact, I just built one not too long ago and I'm getting ready to build another for my daughter. The trick is configuring it legally. This requires what is called an off-list lower receiver.
I don't have the time to go into it now but here is a link to Calguns where you can read all about it.
http://www.calguns.net/
My guess is that Colt owns the trademark "AR-15" designation. That particular rifle would have to be a registered assault weapon in Kalifornia and would be unavailable to virtually anyone who didn't own one during the 90 day period allowed for registration.
Firearms of similar design can be built up without the necessity for registration. Such rifles would operate virtually identically to the AR-15, but would lack some of the "military" features that are now perfectly legal to have in most other states. The key sacrifice is to give up the pistol grip. The challenge is to not run afoul of any of the various "assault weapon" laws.
One can also give up the detachable magazine which then allows the use of the "military" features.
In effect, a gun owner can have ANYTHING they want. They just can't have EVERYTHING they want on the same rifle. To the knowledgeable, the so-called "assault weapons" laws are the stupidest laws on the books and only serve to inconvenience gun owners with absolutely no social benefit whatever.