At any time in the past the U.S. Supreme Court could have ruled that "arms" are defined as single-shot weapons. But that decision would have only applied to state militia weapons.
After Heller, that decision now would apply to everyone. They have already decided (in the D.C. case) that "keep" means "keep in your home", "bear" means "bear in your home", and "arms" means "handguns".
With a magazine capacity limit of 10 rounds.