What to Know About the Type of Gun Used in the Orlando Shooting

4 minute read

Correction appended, June 15

The Orlando shooting has once again put military-style assault rifles back in the spotlight.

Police say Omar Mateen used a Sig Sauer MCX, a semiautomatic assault style rifle that is similar in appearance and capabilities to the better-known AR-15, when he killed 49 people and injured more than 50 others in the deadliest shooting in U.S. history. While Mateen didn’t use the AR-15 itself, the similarities between the gun has drawn attention to the highly deadly military-style rifle, which has been the weapon of choice for several mass shooters. The similarities prompted widespread reports, including by law enforcement authorities, that Mateen used an AR-15 itself, though officials have since clarified that it was the Sig Sauer MCX.

But there’s a reason that the AR-15 is often used as shorthand for assault style rifles. “For practical purposes, for the person that’s just tuning in, the non-gun owner, it’s a very similar type of firearm,” Rob Pincus, who has trained soldiers, police officers and civilians on rifle use for 15 years, said of the AR-15 and the Sig Sauer MCX

Here are facts about the AR-15:

What is the AR-15?

The rifle has been an American staple for decades. Former Green Beret Barry Sadler sang an ode to his AR-15 he used in the Vietnam war in the 1960s, entitled “One Son of a Gun of a Gun.” A line from the song: “You see this AR-15, she’s hot and she’s mean, and she ain’t built for love or fun. And yet this AR-15 is part of the team. She’s one son of a gun of a gun.”

According to the National Shooting Sports Foundation, “AR” does not stand for “automatic” or “assault” rifle. It stands for “ArmaLite” — the company that created the AR in the 1950s. And failed 2015 legislation that would have banned some semi-automatic guns did not classify the AR-15 as an assault weapon.

The AR-15 and the M-16 are identical in shape, style and operation, except for a key distinction: the M-16 is only automatic, said Frank Proctor, a performance shooting and law enforcement training weapons expert in Alabama. Because the two rifles are so similar, the AR-15 is essentially the semi-automatic version—one pull of the trigger, one shot—of the M-16, Proctor said.

Automatic AR-15s do exist, but the vast majority of Americans cannot buy them.

What are the AR-15’s technical specifications?

The weapon’s caliber is .223, meaning the bullet is .223 inches in diameter. While that is smaller than some rifles, it is comparable to an M-16’s caliber—long the U.S. military’s most popular rifle.

The weapon’s projectile velocity—how fast the bullet travels—is around 3,200 feet per second.

Why is the weapon popular with gun owners?

AR-15s are highly customizable, Proctor said. For all AR-15s, the shooting action is standard, “but it can go from there, however you want to accessorize it,” he said. He also said the gun is favored because of its low recoil.

While many AR-15s can be purchased for less than $1,000, sights, grips, stocks and other accessories can cost hundreds of dollars. This 100-round drum magazine costs $116.99.

It is also normally easy to purchase. There is no waiting period for the AR-15 in Florida.

The AR-15 was classified as an “assault-style” weapon and outlawed under the assault weapons ban that lapsed in 2004.

What shootings has the AR-15 been involved in?

The gun burst into the national gun control discussion in 2012 following the shooting of children at Sandy Hook Elementary School in Newtown, Connecticut.

AR-15-style rifles have been used in recent mass shootings at in Aurora, Colo.; Santa Monica and San Bernardino, Calif., and now Orlando.

Correction: The original version of this story incorrectly described the gun used in the Orlando shooting. It was a Sig Sauer MCX, a semiautomatic assault style rifle that is similar in appearance and capabilities to the better-known AR-15.

More Must-Reads from TIME

Contact us at letters@time.com