The Florida School Shooter Used An AR-15 Rifle. Here’s What to Know About the Gun

A school shooting at Marjory Stoneman Douglas High School that left 17 people dead and another 14 wounded in Parkland, Florida on Wednesday has once again put military-style weapons back in the spotlight.

Nikolas Cruz, the 19-year-old suspected shooter, used an AR-15 semi-automatic style weapon during the massacre, law enforcement officials told the Associated Press. The highly deadly military-inspired rifle has been the weapon used by several mass shooters. The AR-15 was most notably used during the 2012 shooting at Sandy Hook Elementary School in Newtown, Conn. that claimed 27 lives, including that of the shooter.

Here’s what are four things to know about the AR-15 rifle including why it’s so popular among gun owners and how many mass shootings the weapon has been used in.

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: M-16s are capable of automatic fire, said Frank Proctor, a performance shooting and law enforcement training weapons expert in Alabama (automatic AR-15s do exist, but the vast majority of Americans cannot buy them.) 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.

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—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 AR-15 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. (Update: On Mar. 9, 2018, Florida’s governor signed into law a bill imposing a three-day waiting limit on firearm sales.)

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.; Orlando, Florida and now Parkland.

Correction, Jan. 15, 2018

The original version of this story misstated the firing characteristics of the M-16. It is capable of automatic fire and other firing modes, it is not only capable of automatic fire. In addition, a bill signed into law by Florida’s governor after this story was originally published imposed a three-day waiting limit on firearm sales.