Well, first of all the feral government is the result of a compact by pre-existing states so I can't see why they would lack the power. But that's beside the point. In this case, they're not "limiting the power and authority of the federal government" so much as insisting that the limits already built in to the Constitution be honored, for the first time in decades. The limit is already there. To gripe at these guys for having the temerity to notice is to shoot the messenger.
Well, I think, and I’m by no means an Attorney or scholar, that the ATF is setup to be a taxing authority. That being the case the Commerce clause comes into play, which has been widely abused. Specifically, I remember the United States v. Lopez, and Rehnquist slapped them down with (IIRC) a precisely worded defeat.