To be clear, the Las Vegas shooting did NOT involve a machine gun. They were semi-automatic weapons that were rigged to fire more rapidly, at about the same speed of fire as machine guns.
ABC News explains:
The federal government started regulating and keeping records of machine guns back when it passed the National Firearms Act of 1934.
That law mandated strict guidelines for manufacturers and put them in place for owners to register their machine guns.
Then in 1986, the feds imposed the Firearm Owners Protection Act which expanded on the original law.
It also banned possession and transfer of new automatic firearms and parts that fire bullets without stopping once the trigger is depressed.
Critically, legal machine guns must be manufactured before May 19, 1986 — the cutoff date imposed by the Bureau of Alcohol, Tobacco, Firearms, & Explosives (ATF).
Because of their scarcity, legal machine guns are very expensive, still require the original 1934 Machine Gun Tax stamp of $200 and the owner or trader must undergo extensive background checks and also permit the federal government to conduct searches.
Law enforcement agencies and the military are not subject to the same stringent measures.
Registered machine gun dealers are also permitted to possess samples to sell to military and law enforcement customers.
State and local governments often regulate machine gun ownership.