My memory is a little hazy on it, but I thought it was the Supreme Court case that established the Constitution as the supreme law of the land and that states couldn't usurp it with their own internal laws.
According to Wikipedia, I was almost right: "state action may not impede valid constitutional exercises of power by the Federal government." Of course, this decision seems to skirt the 10th Amendment, so...
Maybe I'm thinking of another SCOTUS case?
It’s time for Remington to take move their manufacturing plant and 1100 jobs out of New York to a state that actually respects the 2nd Amendment. The hypocrites in Albany don’t mind making and exporting guns to other states, while denying their own citizens the same right.