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.
