All in the interpretation of "Arms" IMO. Some say it is unrestricted in it's meaning and any type of weapon is covered and thus protected. Other would say it has it limitations. I am in the second group, I believe it is not all inclusive in it's meaning just as I believe the 1st A is not all inclusive in it's Freedom Of Speech protection. I have found I am in the minority here on this though.
The NFA is not a perfect balance but I think it is one that is at least near the "middle" of the debate overall. No doubt it could use a "update" and refinement.
Regardless the NFA has stood the test of time and legal challenges so I would imagine it is here to stay regardless of what any of us believe.