Last night in Orlando, a man armed with an assault-style rifle killed at least 50 people and wounded 53 others in a crowded nightclub.

Six months ago, in San Bernardino, Calif., a man and woman armed with assault-style rifles killed 14 people and wounded 20 others at a holiday party.

In 2012, in Aurora, Colo., a man armed with an assault-style rifle killed 12 people and wounded 58 others in a crowded movie theater.

Also in 2012, in Newtown Conn., a man armed with an assault-style rifle killed 28 people and wounded 2 others at an elementary school.

The gunman who killed at least 49 people in a shooting rampage at an Orlando nightclub has been identified as 29-year-old Omar Mateen. (Deirdra O'Regan/The Washington Post)

One common denominator behind these and other high-casualty mass shootings in recent years is the use of assault style rifles, capable of firing many rounds of ammunition in a relatively short period of time, with high accuracy. And their use in these types of shooting is becoming more common: There have been eight high-profile public mass shootings since July of last year, according to a database compiled by Mother Jones magazine. Assault-style rifles were used in seven of those.

In the past 10 years, assault-style rifles have been used in 14 public mass shootings. Half of those shootings have occurred since last June.

Assault-style weapons have long been a flashpoint in the American gun debate. They were outlawed in 1994. But that ban expired in 2004 and Congress opted to not renew it. Gun rights proponents point out that rifles, of any type, are rarely used to kill people in the U.S. Because of that, researchers have generally found that the assault weapons ban had little impact on U.S. homicide rates while it was in effect.

On the other hand, compared to other firearms, assault-style rifles make it fairly easy to kill or injure many people in within a short period of time. So perpetrators wishing to inflict indiscriminate harm on a large crowd of people often turn to them. Of the 10 mass shooting incidents with the highest number of casualties — killed AND wounded — in the U.S., seven involved the use of an assault-style rifle, according to Mother Jones's database.

Early Sunday, June 12, a gunman opened fire on a crowded nightclub in Orlando. He killed at least 49 people. Here's what you need to know. (Monica Akhtar/The Washington Post)

Terrorist groups have taken note of the widespread availability of assault rifles and other guns in the U.S. In 2011, al-Qaeda encouraged its followers to take advantage of lax guns laws, purchase assault-style weapons and use them to shoot people.

"America is absolutely awash with easily obtainable firearms," American-born al-Qaeda spokesman Adam Gadahn said in a video. "You can go down to a gun show at the local convention center and come away with a fully automatic assault rifle, without a background check, and most likely without having to show an identification card. So what are you waiting for?"

Gadahn was incorrect on one point — fully automatic weapons, which shoot continuously when you hold down the trigger, have been banned since 1986. But he was correct on other the other points: Most states don't require background checks for firearms purchased via private sales at gun shows. Most states don't require showing ID to purchase a firearm from a private seller.

Indeed, federal law allows people on terror watch lists to purchase guns, and thousands of them have done so.

The ease of purchasing guns in the U.S., even powerful ones designed to kill many people in a short period of time, is underscored by a crucial fact in Mother Jones's database: Of the 79 mass shootings since 1982 that Mother Jones was able to determine purchasing information for, 63 were committed with guns purchased legally.

Editors note: The headline in this story was changed to make it clear that the weapon involved in Sunday’s shooting in Orlando was not an AR-15. Orlando Police officials first classified the weapon used in the rampage as an “AR-15-type assault rifle.” On Monday, officials said the weapon used was a Sig Sauer MCX. While in many ways similar to the AR-15 family of rifles, the MCX relies on a different gas system to operate and cannot be fairly classified as an AR-15.

The scene in Orlando after a gunman opened fire at a nightclub

An injured person is escorted out of the Pulse nightclub after a shooting rampage, Sunday morning June 12, 2016, in Orlando, Fla. A gunman wielding an assault-type rifle and a handgun opened fire inside a crowded gay nightclub early Sunday, killing at least 50 people before dying in a gunfight with SWAT officers, police said. It was the deadliest mass shooting in American history. (AP Photo/Steven Fernandez)