But California cannot exceed Federal Law, or supercede the US Constitution; that would be the basis for a lawsuit all the way to SCOTUS on this in my view.
Besides, how would they possibly enforce it?? How many semiautomatic military style battle rifles have come into the State since the ban was lifted?? How much ammo do those owners have stockpiled??
I dunno about you, but I would not want to be the State employee tasked with trying to round all those weapons up...
Which federal law? And at any rate, the states are generally free to exceed federal law unless a reviewing court finds that (a) Congress passed a comprehensive regulatory regime intended to cover the field and (b) either (i) Congress expressly or impliedly prohibited states from going beyond the limits of the federal law or (ii) allowing states to do so would wreck the regime set up by the federal law. Also, you have issues of comity between sovereigns and in an area where the Constitution has not given Congress full power to legislate, and has not wholly removed that power from the states, the states may legislate with impunity within the sphere allowed to them and not denied them by virtue of the grant of power to Congress.
Also, I doubt if the Second Amendment would be read as providing a right to own a semi-automatic weapon as opposed to any of the plethora of other weapons that aren't semi-auto. As far as I recall, the Supreme Court's holding on the Second Amendment essentially says that the states cannot wholly ban the possession of all firearms; that is a far cry from saying that the states cannot limit which firearms may be possessed. After all, if there's a Constitutional right to a semi-auto, can it be gainsaid that there is also a right to a fully-auto, or even a machine-gun, or military grade firearms?