JUDY WOODRUFF: While the shooter’s motives remain unclear, we are learning more about the veritable arsenal that this man brought into his hotel room.

William Brangham explains how some of those weapons were likely modified to become even more deadly.

WILLIAM BRANGHAM: You can hear it in those horrible cell phone videos from Sunday night.

(GUNFIRE)

WILLIAM BRANGHAM: That rapid fire is virtually impossible for one person to do, unless you’re using a fully automatic weapon.

Fully automatic means one pull of the trigger fires a continuous stream of bullets. It continues firing until you release the trigger or run out of ammunition. That’s certainly what the video from Las Vegas sounded like, but it’s been illegal to sell automatic weapons since 1986, when Ronald Reagan signed a law that banned them.

They were simply considered too deadly for civilians to own. Existing owners in most states were grandfathered in, and those can be sold, but no new sales to civilians have been allowed since.

So how was the killer able to shoot so many rounds so quickly? One clue is right here. This is one of his guns from that hotel room. See this part of the gun? That’s an added modification known as a bump-stock, and it’s likely one of the ways he was able to kill so many people so quickly.

A bump-stock is one of several ways that people now modify legal semiautomatic weapons into acting like a fully automatic machine gun. YouTube is full of videos of manufacturers and their customers showing how these easy inexpensive bump-stocks actually perform.

If you attach a high-capacity magazine, like this one that holds maybe 100 rounds, these weapons become virtually indistinguishable from automatic weapons.

Another common modification is the so-called gat crank, where this small silver crank is inserted into the trigger mechanism of a semiautomatic weapon, making it act like a fully automatic one.

These current modifications are not technically illegal, because, remember, a gun is only considered fully automatic if one pull of the trigger unleashes that continuous volley. These devices don’t do that. They just dramatically speed up the actual firing mechanism of the gun. So they can still be legally sold to anyone under federal law.

In fact, they’re available right now at Wal-Mart and at sporting goods stores and all over the Internet.

For the PBS NewsHour, I’m William Brangham.

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