For example, if the particular issue addressed by the state law is also addressed by the federal law, there's the question of whether the particular federal law was intended to "occupy the field", i. e., be the final word on the subject. In that case the federal law preempts the state law and applies instead of the state law.
It seems to me it would be hard to argue this. If it were easy, California couldn't require a DROS in addition to the 4473 form. And a felon could own a muzzleloader in any state.