You forgot to explain why so few machine guns are in circulation.
Because criminals don't need them to rape women, they don't need them to commit armed robbery and the don't need them to kill their gang rivals....notice, however, that in Mexico...where you have only one gun store in the entire country, controlled by the Mexican military, it is on a military base, the Mexican drug cartels have all the fully automatic weapons that they want. Why? Because they need them to deal with the other drug cartels as they fight for control of drug territory in the rural areas of Mexico......
If American criminals needed fully automatic weapons to conduct their business, they would have fully automatic weapons.....