That allowed him to spray bullets into the crowd of 22,000 people.
A U.S. law enforcement official, speaking on condition of anonymity, told Reuters authorities believe 64-year-old Stephen Paddock had at least two such weapons when he fired from his 32nd-floor hotel room, killing at least 59 people and injuring more than 500.
The rat-a-tat sound of the Las Vegas shooter's gunfire prompted police at the scene to report thegunman was using an "automatic" weapon - a term often used to describe a fully automatic gunthat can fire as many rounds as its magazine, drum or belt holds by pulling and holding the trigger.
Those weapons have been largely outlawed for three decades, though Paddock could also have used legal or illegal means to alter semi-automatic rifles, which fire a round every time the trigger is pulled.
"From the audio, that is not someone who has a traditional semi-automatic rifle firing it in its normal condition," said David Chipman, a former Bureau of Alcohol, Tobacco, Firearms and Explosives agent who advises the gun control group Americans for Responsible Solutions. "Either it's a machine gun or it's been modified. I don't believe a human can do that with his finger."
Police said they recovered a total of 42 weapons belonging to Paddock, including 23 from the hotel room and 19 at his home in Mesquite, a small desert town about an hour from Las Vegas. Some were automatic weapons or semi-automatic rifles illegally modified into fully-automatic weapons.
It was not clear when or how Paddock obtained the guns. Chris Sullivan, owner of the Guns & Guitars shop near Paddock's home in Mesquite, Nevada, confirmed Paddock had legally purchased firearms from the store but did not offer more detail.
The shooting has prompted renewed calls from Democrats for tighter gun laws, though it is unlikely the Republican-controlled Congress will take up such measures.
In 1986, Congress barred civilians from buying or selling fully automatic weapons made after that date, though individuals can legally possess older weapons after passing a background check and obtaining a special permit.
There are about 176,000 pre-1986 machine guns registered with the U.S. government that can be legally transferred, and they typically cost tens of thousands of dollars.
But there are also less expensive legal products that can allow semi-automatic rifles such as the AR-15, which are much more widely available, to fire hundreds of rounds per minute.
A "bump stock," for example, replaces a semi-automatic rifle's stock, which rests against the shoulder to provide stability and absorb recoil.
The bump stock causes the gun's recoil to press against the shooter's finger after each shot, firing rounds much more quickly than possible by pulling it manually.
The website for one such product, Slide Fire, shows several videos in which shooters launch multiple rounds per second in bursts that sound almost indistinguishable from automatic fire. Reviewers have reported that the product permits a shooter to go through hundreds of rounds every minute.
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