California Options;
I can't condone this option due to my career choice, but as FREE MEN you ALWAYS have the option, right or wrong, to not comply. Just understand the consequences.
Install one of the newly designed "magazine lock" devices, and hope they are considered "legal".
*NEW* Prince "Freedom" Bullet Button
AR Mag Lock
Franklin Armory DFM conversion
Make a "Featureless" rifle; by removing a telescopic/folding/thumb-hole stock, pistol grip, foreword vertical grip, bayonet lug, grenade/flare launcher and flash suppressor (basically, making a Mini-14 or M1a) you can now have a semi-automatic, center-fire rifle, with a detachable magazine. As long as the "fixed" magazine is 10 rounds or less, and the overall length of the rifle is at least 30".
This is where the "new" stocks come from, and all the kydex pistol grip wraps. The biggest companies making products are;
Thordsen Customs
Exile Machine
Hera Arms (but I don't think the CA version has been released yet, as it's not listed on their site.)
Register the rifle as a "California Assault Weapon"..... The only problem with this is that the criteria for "registering" has not changed. To date, only two "groups" of persons are allowed to register an AW in Cali, so if you don't belong to one of these groups... good luck!
A full time peace officer WITH a signed letter from his/her Chief of Police authorizing the registration.
A License Hollywood Prop designer/caretaker (who can be audited regularly, without notice, by CA-DOJ).
Needless to say, the options aren't "great" but there are a few.... and more and more are coming out every day.