Federal court? To quote Andrew Jackson: “The decision of the supreme court has fell still born, and they find that they cannot coerce Georgia [Montana/Tennessee/Missouri] to yield to its mandate.”
All the federal alphabet soup agencies show up to arrest the guy from Montana who built a machine gun.
The local state authorities will do nothing, or will be arrested also. Or if things get real stupid, shot.
Remember, Jackson had control of the executive branch of the Federal Government. The state of Montana (or any of the other states that have passed similar measures) does not. They do not even have the power to tell the EPA to shut up, much less the BATF.