Though as alluded to previously, the Montana Act has not been regulated favorably by both the BATFE and the Federal courts. I have no idea as far as rulings and current status/production/licensing of "NFA"intrastate weapons (a paradox, I know) in states with similar FFA laws, like Tennessee.
Seeing as how the feds are doing absolutely nothing against states that have legalized marijuana, if they did try prosecuting someone who sold/purchased/used/manufactured what would have been a regulated firearm wouldn't there be grounds for some "equal protection under the law" defense/lawsuit?
I not a lawyer, nor did I stay at a holiday inn express last night, but I would think the government picking and choosing what laws to enforce (not that there isn't a lot of that already) shouldn't fly very far.