Montana has passed a law declaring that firearms manufactured in Montana, for use in Montana, and never leaving Montana, are exempt from the myriad federal regulations governing such things since no interstate commerce is involved.
Per Wickard v Filburn, if you grow corn on your own property, to feed to your own livestock, you are affecting interstate commerce, since the corn would otherwise have been purchased from someone who might possibly be from out of your state, therefore, under the commerce clause of the constitution, the federal government has the right to regulate your growing of the corn.
That was the logic of the ruling, back in the 30s to enforce farm production limits at the federal level. Obviously the same logic would apply to the manufacture of anything at all, or so I expect the feds would argue.
When marijuana was legalized for medicinal purposes in, I believe Washington or Oregon, the Feds came in a busted the sellers, although the practice was legal at the state level. I don't know if that case was completely settled or not, but Pres Obama has announced that the justice department will, in the future, not pursue such cases.
Manufacture your own firearm in Montana, and dare the feds to come and get it. Assuming you survive the experience, you will then have standing to spend the next 10-20 years working your case up to the supreme court. By that time, the court will have changed substantially, but who knows in what direction. We may be in Carrie Prejeans first term, or Obamas 5th.
Whatever happens, bring lots of money, and lots of patience.