We all know the U.S. of A. has some interesting laws to control gun ownership. The law is different in all fifty states. Therefore it is difficult to make a nation wide comment about state laws.
But I can explain what happens at the national (i.e. federal) level. The U.S.of A. Constitution´s Second Amendment says that "because a well armed militia is necessary, the right of people to bear arms shall not be limited." That´s it all, more or less. This amendment is what allows Americans to own and use weapons of all kinds.
For example, the Waco massacre got started when the authorities tried to carry out an investigation at David Koresh´s camp. Koresh was said by his followers to be the second coming of Christ. This guy made a living from his folowers´donations and selling weapons, including .50-caliber machine guns like this
.50 caliber machine gun (from walkingdead.wikia.com)
Because the second amendment is so short and lacks detail, and in the U.S. of A. there is a tradition for people to own weapons, some states have laws which allow some individuals to use heavy weapons. For example, here is a man who will do a demonstration of his personal 105 mm cannon. Look at the size of the bullet.
Old man demonstrating a 105 mm cannon
I wonder, why would a guy be interested in something like this? Those rounds must be very expensive, and it sure seems a bit exaggerated to shoot a stranger at long distance with a 105 mm cannon. But the way this works, that cannon´s owner can blast you to kingdom come if you enter his property without asking for pemission (In some states he has to post signs saying something like “Do not enter, if you do the owner will shoot at you with a 105 mm howitzer”).
I don´t like to live near someone like that. The Lord knows he looks sane, but hey, one mistake and there goes MY property. Let me show you an example: here we see him with his assistant preparing to fire that cannon of his:
Old man preparing to fire a 105 mm cannon
And here we see the results of this demonstration. The cannon round flew over 5 km and fell quite close to a car stopped in the old man´s property. This means it was quite legal for the round to hit the car and blow it apart.
Round strikes the ground near a parked vehicle
What else can I say? To live in a place where people are armed like that you have to be crazy. Or at least one needs a very well-fortified house, as we see below. Because if an old drunk comes around and starts shooting a .50 caliber machine gun or a howitzer, a house with extremely thick walls and roof sure comes in handy.
House with very thick walls and roof