If I wish to purchase a 50cal machinegun and practice firing it and don't have a criminal record or history of mental illness or even poor eyesight, what business is it of your's?
Because when you have a gun that big you become a danger to those around you as well.
I feel it is my business if my country feels like it should give out heavy weaponry with no more restriction than "amendment, and your mind looks ok", especially when that weapon can go through cars or reinforced walls, and even cleanly through a tank. We have trained, federal professionals handle those weapons because they are designed to cause mass damage ontop of death.
While I get the car reference, there is a differences between a car and a gun. A car is primarily designed to get you from place to place with the chance of killing something while a gun is primarily designed to cause harm/kill the thing it is pointed at. One is a mode of transport, while the other is a tool designed to kill. You can target fire, but the primarily reason for its existence is to cause harm.
Assault Rifles and full-auto weapons are trouble because the spread and uncontrollability of such a weapon can hit civilians, go through walls and hit non-combatants. Sawn-off shotguns can be hidden, and the increased spread can hit more non-combatants. Grenades and other high explosives hurt without prejudice.
The second amendment gives people the right to bear arms. The vagueness could imply every weapon in existence up to atom bombs, while it can also imply defensive arms and not a stash of grenades.
Maybe people can be trusted with a full auto weapons enough to sell them at K-Mart, but personally I'd rather crime be performed with small arms and knives and have it be a -little- difficult for them to get their hands on heavy weapons than have the government personally give bigger weapons to the populous.
But while people can be trusted or distrusted, a bullet can and does hit without prejudice. A sawn-off shotgun spread, .50 cal bullet, full-auto spray bullet, etc fly without prejudice.
Call me paranoid, but I trust the average citizens as much as I trust our government, and I trust a .50 cal bullet even less.