https://www.washingtonpost.com/news/checkpoint/wp/2017/10/02/video-from-las-vegas-suggests-automatic-gunfire-heres-what-makes-machine-guns-different/?utm_term=.6434f4069d30

While much remains unknown about the specific weapons Stephen Paddock fired from the 32nd floor of the Mandalay Bay Resort and Casino in Las Vegas, killing at least 58 people and injuring potentially hundreds more, people familiar with the investigation said Monday that about 20 firearms were found in the gunman’s room.

The sound of gunfire captured on videos recorded during the massacre indicates that Paddock used at least one automatic weapon, a type of firearm that is highly regulated in the United States though a fairly common find at gun shows and firing ranges, including one near Las Vegas that allows customers to shoot military-grade weapons.

It was unclear how Paddock would have acquired an automatic weapon, if he did use one. Those weapons can be legally owned if made before May 1986 and registered with the federal government, or owned by licensed dealers.

So far, at least one has been identified as an AK-47 type rifle, outfitted with a stand to steady it and improve accuracy, said people close to the ongoing probe. Investigators are working to determine if Paddock modified mechanical components to a semiautomatic rifle to make it fully automatic, which is illegal, or if he used a legal modification like an attachable crank, which depresses the trigger faster than a finger and can be bought online for as little as $40.