Those weapons have been largely outlawed for three decades, though Paddock used a device to give him “a souped-up semi”.
Jill Snyder, special agent for the U.S. Bureau of Alcohol Tobacco and Firearms (ATF), said 12 of the guns found in the gunman’s hotel room were fitted with so-called “bump-stocks”. The device basically replaces the gun’s shoulder rest, with a “support step” that covers the trigger opening. By holding the pistol grip with one hand and pushing forward on the barrel with the other, the shooter’s finger comes in contact with the trigger. The recoil causes the gun to buck back and forth, “bumping” the trigger.
Technically, that means the finger is pulling the trigger for each round fired, keeping the weapon a legal semi-automatic.
Is that you’ve got to be very detailed about the regulation and even then often enough someone will engineer around it. Regulation, therefore, isn’t as effective as the regulators assume.