There is a line but full auto guns do not cross it. The debate about tanks etc goes into what organized militia's should have access to or rather whether or not the government should have these weapons maybe. Full auto machine guns are very effective at killing crowds of people standing close together, but you can't stop those kinds of massacres since people can simply run over crowds with a truck. The Paris truck attack killed like 80 and injured 400. So banning full auto guns doesn't really do anything other than make citizens less equipped to keep the government in check. I believe every state requires background checks for full auto guns. Full auto guns are pretty hard to obtain but that's mostly because they are very expensive.

The states with the highest gun ownership have very low gun murder rates. Often much lower than other states. The only places in the US with high gun crime are areas with higher populations of black people. If you compare race demographic maps to gun ownership maps every state with almost all whites has next to 0 gun crime even if the gun ownership is very high. There isn't much of a difference whether a state is conservative or liberal. Population density doesn't even seem to be that much of a factor. From what I can tell gun crime is pretty much just a black problem in areas with poverty stricken blacks. Cities like New York City that are very mixed race have pretty low gun crime, probably because the people there all are rich. Point is % of gun ownership doesn't really matter, it's an issue with certain people. I suppose we could wall off cities like Chicago. I'm guessing sweden and finland have mostly white people.