A rack of rifles at Firing-Line gun store in Aurora, Colo., in 2013. (Ed Andrieski/AP)

Law enforcement officials say that many of the guns recovered after the deadly Las Vegas mass shooting Sunday night were modified with legal accessories known as “bump stocks” to achieve rapid rates of fire.

To understand what these devices do, you need to know a little gun terminology. Most guns sold in the United States today, even military-style rifles like the AR-15, are semiautomatic. They fire a single bullet with each pull of the trigger. Fully automatic weapons, which fire a continuous stream of shots when the trigger is depressed, are for all intents and purposes illegal for civilians to purchase or own.* (See below.)

In recent years, companies such as Slide Fire and Bump Fire Systems began manufacturing devices that allow semiautomatic weapons to simulate fully automatic fire. The devices work by allowing a gun's recoil energy to rock, or “bump,” forward and back, rapidly pushing the trigger against the shooter's finger and firing a single round each time.

The result: an effective rate of fire approaching the equivalent of hundreds of rounds per minute. Here's one in action:

Because guns modified this way still fire only one round with each pull of the trigger, the Bureau of Alcohol, Tobacco and Firearms has ruled that they are legal.

After Slide Fire submitted one of its stocks to ATF for review in 2010, the bureau noted that “in order to use the installed device, the shooter must apply constant forward pressure with the non-shooting hand and constant rearward pressure with the shooting hand. Accordingly, we find that the 'bump stock' is a firearm part and is not regulated as a firearm under Gun Control Act or National Firearm Act.”

It's also worth pointing out that, technically speaking, you don't need special equipment to bump-fire a gun. Experienced shooters can achieve the same effect by loosely holding a gun in a certain way, but this sacrifices accuracy.

Part of the appeal of bump stocks is that they're more accurate than traditional bump fire methods. “The patented Slide Fire® stock allows shooters to safely bump fire their rifles without compromising accuracy,” Slide Fire states on its website.

Recreation is the other main draw. Slide Fire's website previously promised “an exhilarating experience that keeps you smiling for days.” One of its promotional videos features exhilarated “first-time shooters,” while in another a voice-over recites the names of Founding Fathers over a backdrop of dramatic music while a man rapidly fires a modified rifle.

Bump stocks are relatively inexpensive and widely available. Some of Slide Fire's models sell for less than $200, with some competitors' models even cheaper. Until a few days ago they were available online at Cabela's, a major outdoor retailer, which marketed them as a way to “maximize your fun.”

In the aftermath of the shooting, which killed 58 people attending a country music concert, the products are under renewed scrutiny. In a news release Tuesday, Sen. Dianne Feinstein (D-Calif.) said bump fire stocks create an “automatic weapons loophole” and that she was looking into legislation to close that loophole.

Slide Fire did not respond to requests for comment.

* There's one major exception to this rule: Automatic weapons manufactured before 1986 can still be bought and sold. But because of their limited numbers and the special permits required to obtain them, they tend to be prohibitively expensive and are primarily collector's items.