Honestly, it doesn't make sense to put anything under NFA. Silencers are harmless, SBR's are bigger than handguns, and full auto is going to be less effective in the hands of a nutcase than semi auto. There is really no sane justification for it at all.
I'm fairly pro gun but I feel slightly uncomfortable with random people with crew served weapons, mortars, GMGs, etc. Although I trust most random people with $50k to spend more than I trust police departments with weapons like that.
There's an upper limit, I'd say. For me, it's a .50 cal M2 or similar machine gun. I don't think those should require extra paperwork. If you want a 20mm or a destructive device, I can understand regulating that. The 2nd Amendment's intent is served well by free access to small arms, I would say. That is enough.
Pat Buchanan came up with a more permissive metric, "anything that doesn't require a trailer hitch", although I can't see there ever being support for indirect fire weapons like mortars.
However, if an armed citizenry is supposed to keep the government in check, why not HEAT warhead anti-armor weapons? E.g. in the context of this discussion: https://news.ycombinator.com/item?id=6186569 which touches on the widespread police procurement of light armor.
