I can't think of any good reason that we are selling machine guns to civilians in the United States. These types of weapons are obviously not needed for hunting purposes, and for self defense a smaller gun can do the job just as well. If it is even possible we can save lives by taking them off the market, then it needs to be done.
Machine guns made prior to 1986 which are in the NFA registry cost no less than $7,000 + $200 tax stamp, and you must go through an extensive background check by BATFE and the FBI which takes between 6 months to a year. Then, the transfer must take place through a Class III gun dealer. This is before you can pick up the old pre-86 machine gun you paid for.
AR-15's are not machine guns. They are semi-auto, meaning one shot for each trigger pull. They cannot be legally converted to be machine guns either. Just because a gun looks similar to a machine gun doesn't make it one. That's just like saying a Fiero Ferrari kit car is the same as a genuine Ferrari.
So, rest assured that no machine gun deals are going down in the parking lot of WalMart. And, for those who actually own a rare registered machine gun - I can assure you that they are not going to use their $15,000+ machine gun to knock off a gas station or commit any other crime. That would be like using a genuine Picasso painting for toilet paper. Also, not one single person has ever committed any crime with a registered machine gun. Not one. Ever.
If you are going to own a gun responsible then by all means go ahead and own that gun. If the government wants to enforce background checks I personally have no problems complying as I have nothing to hide. To be specific if you are a stand up civilian and have no violent past then by all means then you should be able to buy a gun regardless of the variety. In response to the question however The US does no allow the sale of "Machine Guns" the US allows the sale of "Assault Rifles" which are not machine guns, but instead rifles that have a full auto capability. A true machine gun is an MG-42 or something similar.
Individual are responsible for their own actions. Just because one uses a car to kill and another uses fire while yet another uses bare hands to beat someone to death. The weapon of choice is not to blame. The person who commits the crime is to blame. It could just as easily have been a baseball bat that killed a person on a dark street as a gun. Baseball bats make cheaper weapons as do matches, lighters, fists etc.
Anything can be bought legally or illegally, making an item illegal is not going to slow down sales. It is just making more laws for the sheep to follow and keep the yoke around their necks. Nothing will stop the sale of guns, the key would be to stem the NEED for guns.
Criminal elements and humans intent on doing harm to others will always find a way to get a gun. If they exist, then someone, somewhere, will offer it for sale and someone, somewhere will buy it. The only alternative to completely eliminate the commerce of gun sales would be to eliminate the supply, and that's not going to happen!