Prohibit the possession of "large-capacity military-style magazines" (whatever that means) that hold 11 rounds or more. "If passed, California would join New York, New Jersey, Hawaii and The District of Columbia in banning possession of these military-style clips." (Note that a magazine isn't a clip, and New Jersey's magazine capacity limit is 15 rounds.)
Treat every ammunition sale like a gun sale (the FBI's NICS background system is so overburdened already, that the FBI has suspended its appeals process because of a manpower shortage).
Define a clear "firearms relinquishment process" for those convicted of a felony or a violent misdemeanor.
Require firearms owners to notify law enforcement if their firearm has been lost or stolen. Stores would also have to report any stolen or lost ammunition.