That is the very issue we are dealing with in California right now. The CA Attorney General argues that the 2nd. Amendment only applies to the feds and not the states. He is asserting "States' Rights". Therefore, since California does not have an RKBA amendment in the state constitution, the state is then free to restrict arms and ammo as it pleases.
Right now there is a movement to put a RKBA into the California State Constitution. I am not hopeful of its passage and if it does pass, the courts will most likely strike it down. The feds and the states need to be compelled by the electorate to stop the cynical ambiguity of whether the Constitution upholds the rights of the people, or those in power, be they fed, state, county, or dog-catcher.
I won't hold my breath.