An AR-15 once again made an appearance at a mass shooting, this time at a Parkland, Fla., high school on Wednesday. The suspect in the shooting, 19-year-old Nikolas Cruz, purchased the semiautomatic rifle about a year ago, according to a law enforcement official.

These AR-style rifles have appeared in some of the deadliest shootings in the past few years, including a concert in Las Vegas, a nightclub in Orlando, a church in Texas and an elementary school in Newtown, Conn.

Here’s what you need to know about its history and use.

What does “AR” in “AR-15″ stand for, and what are its origins? 

“AR” doesn’t stand for assault rifle. It stands for the Armalite rifle, named after the company that developed the weapon. It was first used during the Vietnam War as an alternative to the M-14 rifle, which was heavy, difficult to control and outmatched by the AK-47. In the late 1950s, the gun manufacturer Colt purchased the rights to the rifle but had difficulty selling it to the U.S. military.

Then-chief of the Air Force Gen. Curtis LeMay took a liking to the weapon after a Colt salesman offered him a chance to shoot watermelons with the gun at a Fourth of July celebration. LeMay ordered 80,000 of them but was rejected by multiple government agencies as well as Congress, which didn’t want to spend money on a new weapon when the M-14 was already in production. LeMay continued to press for its use and even appealed to President John F. Kennedy (who rebuffed him).

In the 1960s, Defense Secretary Robert McNamara halted the production of the M-14, and the rifle finally made its debut on the battlefield in Vietnam as the M-16 assault rifle.

What are defining characteristics of the rifle?

The military’s M-16 was originally fully automatic, meaning it fired several rounds with each pull of the trigger. Its civilian counterpart, the AR-15, is semiautomatic — the user needs to pull the trigger to fire each shot.

The AR was designed for speedy reloading in combat situations, and it can fire dozens of rounds in seconds. The butt of the rifle, or the stock, has a large internal spring that absorbs the shock of each firing. The low recoil makes it easier to shoot and is more accurate than earlier military weapons. It can also be easily customized by adding scopes, lasers and more.

Who can purchase an AR-15?

It depends on the state you live in. In Florida, an AR-15 can be bought by anyone older than 18 with a clean record. There is no waiting period. (Handgun purchases typically require a three-day waiting period for anyone older than 21.) It is legal to own fully automatic weapons, but they are heavily regulated. Some states prohibit ownership of semiautomatic rifles with certain characteristics, such as the AR-15.

What are the laws surrounding assault weapons?

Gun advocates maintain that semiautomatic weapons such as the AR-15 should not be classified “assault weapons” because they are not fully automatic and because the guns have recreational uses, such as hunting and target shooting.

Yet gun-control advocates say that distinction is arbitrary and that the weapons are just as dangerous because they are designed to kill a large number of people quickly. They often point out that the AR-15 has a high muzzle velocity, which, combined with the small .223 round, produces a violent ricochet through an animal body if it hits bone.

Bolt-action rifles with cartridges loaded in 30.06, a common deer-hunting caliber, fire a round that travels slower with more blunt force, though the muzzle velocity varies for lighter and heavier rounds.

In 1994, an assault-weapons ban signed by President Bill Clinton outlawed the AR-15. But the law had a lot of loopholes, and gun manufacturers circumvented it by modifying the weapons. The ban expired in 2004, and sales of the gun increased during the George W. Bush and Barack Obama administrations. The National Rifle Association labeled it “America’s most popular rifle.”

Lawmakers were not interested in picking up the effort to ban assault rifles until 2012, when Sen. Dianne Feinstein (D-Calif.) introduced legislation to ban assault weapons following the massacre at Sandy Hook Elementary School in Newtown, Conn. The effort eventually failed.

Now, gun-violence experts want to see the 1994 ban restored, and lawmakers are calling for new legislation. A new bill introduced by Feinstein and supported by 22 other Democratic senators would ban selling and manufacturing 205 “military-style assault weapons.” The bill also calls for a ban on high-capacity ammunition magazines.

Alex Horton contributed to this report.

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