Police have said there were 23 firearms in the Las Vegas killer’s hotel room, though have yet to describe in detail the weapon or weapons used in the Oct. 1, 2017, shooting.
At PolitiFact, we won’t speculate and will update this post as we learn more.
But the shooting, where more than 50 people were killed and more than 500 injured, has again highlighted a distinction about firearms that is important to know: the difference between automatic and semi-automatic rifles.
In simplest terms, "semi-automatic" refers to any firearm designed to fire one bullet with one trigger squeeze, then automatically reload the chamber with a cartridge from a magazine and be ready to fire again.
The term applies to a whole range of modern firearms, from hunting and target rifles all the way up to so-called black rifles that look like what a soldier would carry. Gun control arguments often focus on the black rifles, but the differences between those and any other semi-automatic rifle often are only cosmetic. Semi-automatic guns all largely operate the same way.
Automatic weapons, which are often described as machine guns, are different, in that squeezing the trigger once fires cartridges repeatedly until the shooter releases it.
While semi-automatic rifles are widely available, fully automatic weapons are not. You can still buy an automatic weapon, but their sale and ownership is highly regulated and exceptionally expensive.
Automatic weapons sales have been restricted in the United States since the 1934 National Firearms Act was passed. Regulations put in place in 1986 made it much more difficult for civilian buyers to get an automatic weapon.
You can still purchase an automatic weapon, because existing guns manufactured before May 19, 1986, were grandfathered in. That amounts to somewhere around 300,000 registered automatic weapons, which can cost thousands of dollars apiece because of their limited availability.
Generally speaking, manufacturers haven’t been allowed to build new automatic rifles to sell to the public since then.
To buy a fully automatic rifle, a prospective owner must pay the Bureau of Alcohol, Tobacco and Firearms $200 and pass a federal background check that shows no record of domestic violence or felony convictions. The process can take months.
Some states like California, Iowa and Kansas, ban private ownership of automatic weapons under any circumstances. But many states, including Nevada, allow it as long as the owner has complied with federal regulations.
"Most people can buy machine guns in lots of states," Michigan-based lawyer and firearms expert Steven Howard has told PolitiFact. "But, and this is one of those classic big ‘buts,’ they have to get through a background check by FBI that is as thorough as if you are getting clearance to become a federal agent."
From 1994 to 2004, there was a federal law that banned the sale of certain types of new semi-automatic weapons, including some types of semi-automatic rifles. It also set a limit on high-capacity magazines. The law was adopted to last for 10 years and was not renewed by Congress when it expired.
We don’t yet know whether the person police have identified as the shooter, Stephen Craig Paddock, 64, of Mesquite, Nev., had used or legally purchased an automatic rifle. But it appears the gunman had access to another option.
Police said he had altered 12 weapons with a device called a "bump stock," which would allow a semiautomatic gun to fire at a rate similar to an automatic. Authorities have not provided further details of the 23 guns in the hotel room, including how many were rifles or handguns, or what kind of guns they were.
Although modifying a semi-automatic gun’s mechanics to allow automatic fire is illegal, bump stocks or slide stocks are not against the law. Instead of altering a gun’s internal mechanisms, they harness a weapon’s recoil to cause the user’s finger to squeeze the trigger repeatedly, sending out fire at a rapid pace.
These government-approved devices allow semi-automatic rifles to mimic machine gun fire.
There are other legal options for achieving the same effect. A device known as a trigger activator is essentially a crank or similar device that fits into the trigger guard and depresses the trigger repeatedly.
"There are workarounds you can find pretty easily on the Internet," UCLA law professor Adam Winkler said. "They tend to be hard to use, but they’re available."
Editor's note, Oct. 4, 2017: This story was updated with police information on the number of weapons, some of which were equipped with bump stocks.
Sources are linked in the story.