From NRA-ILA:

Senate Bill 249, as amended, would make a small but profound change to the definition in state law of what constitutes a detachable magazine for a semi-automatic firearm.  By doing so, as many as one million semi-automatic rifles (with a “bullet button” or other magazine locking device), which were legally sold in California over the last twelve years, would become illegal on July 1, 2013.  If you’re one of the hundreds-of-thousands of Californians who purchased one of these modern sporting rifles (a centerfire semi-automatic rifle with a magazine locking device that prevents detachment of the magazine without a bullet or tool) since 2000, your firearm will have to be surrendered or may be confiscated.