I don't have a perfect knowledge of the law. I'm also oversimplifying. Bear with me.

Back when the second amendment was created, it was entirely reasonable that citizens could arm themselves and realistically overthrow their policing force and government. It's a large part of how the US was able to gain it's independence.

I don't think that is reasonable today - weapons for waging modern warfare do more to infringe the rights of others when owned by civilians. Also, US citizens would not stand a chance in armed combat with the military.

I like drawing the line with a police force because I think police should be made up of citizens (not soldiers). Policing a population and national defense are totally different objectives that should not overlap. There should be a clear, consistent line where on one side of the line the police are deployed to enforce the laws of the locality and on the other side the military is deployed to protect the nation. If the police force deploys weapons that the military uses for national defense, that line is very blurry and citizens can be oppressed without the use of the military. It should be a very explicit, unambiguous action for a government to use military force against it's citizens.

A person that breaks the law by murdering someone and then taking a hostage is not a national defense concern and military tactics and weapons are not necessary.

A person that makes meth and sells it on the street, is not a national defense concern. It may be a concern to the society that this happens in, and the community should police this behavior (if it wants), but military tactics and weapons are probably not necessary here either.

There is a lot of grey area here. I'm sure there are edge and corner cases that are not so clear. I don't have all the answers. It does not bother me that G.I. Joe has tanks, grenades and automatic weapons. It very much bothers me that Jane the Street Cop could have semi-automatic rifles, flash grenades and body armor between 9-5, but I could not. To be clear, I don't want to de-arm the police.