Depending on the state, there are basically 5 conditions you have to meet for a self defense claim in court.
1 – You cannot be the aggressor, or you must be the “innocent party”
2 – The threat has to be immediate
3 – The amount of force you use has to be proportionate to the threat
4 – Some states, duty to retreat
5 – Your conduct has to be reasonable
So the two that “could” become issues are items 3 & 5. If someone shoots a shot at your house, and you melt his car down with a full belt from your M60, something tells me they’re going to have an issue with that.
I don’t think the NFA thing by itself is the issue…that typically doesn’t change the circumstances of a shooting. However if you use of said NFA device is gratuitous or inappropriate, then you’re going to have problems.
“I think there’s a right-wing conspiracy to promote the idea of a left-wing conspiracy”