If the line is not drawn on full auto, where is the line separating Arms from Weapons of War? It sounds like some of you are saying there is no line... Is that what you are saying?
The "line" as I see it should not be drawn at full auto and >50 caliber, but at explosives and weapons of mass destruction. The test for how far a right can be reasonably regulated is how much public interest does said regulation serve vs. how much liberty does it take away. In my opinion, the current level of regulation on full auto and large-caliber weapons does not represent a large enough increase in public safety to justify the degree of liberty that it strips away.
For example, since the passage of the NFA there have been numerous violent crimes committed with full auto firearms, but of all those crimes only one murder was committed with a legally registered machine gun. The fact of the matter is that most classes of weapons regulated by the NFA are easy enough to either obtain on the black market or fabricate that it does not act as an impediment to an even mildly resourceful criminal. No more graphic example can be found than the North Hollywood Shootout as the two bank robbers in that incident used not purpose-built machine guns, but illegally modified semi-automatic rifles. Criminals can illegally obtain or manufacture machine guns easily enough that only people who abide by the law are affected by the NFA.
The reason that explosives and weapons of mass destruction belong in a different class is because their destructive power is so great that they're extremely difficult, if not impossible, to use without causing extraordinary amounts of collateral damage. There is almost no way that a person could use a hand grenade or canister of VX nerve gas in self-defense without posing a significant risk to the safety of innocent bystanders. With a moderate amount of training, however, a machine gun can be used without representing a significantly greater risk to bystanders than most other common firearms.
Honestly, I think that the difference between a machine gun and a weapon of mass destruction is pretty obvious to most people. In fact, I view the mention of such extreme examples of "arms" in such a discussion to be something of a straw man.