A Las Vegas concert. An Orlando night club. An elementary school in Newtown, Conn. A Texas church. And now a high school in Parkland, Fla.

America's most popular weapon was there for all of them.

AR-15-style rifles have increasingly appeared in American mass shootings, including the deadliest high school shooting in the nation's modern history at Marjory Stoneman Douglas High School in Parkland, Fla., Wednesday.

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The National Rifle Association has called the AR-15 the "most popular rifle in America" and estimates Americans own more than 8 million of them.

The NRA said "the AR-15 has soared in popularity" because it's "customizable, adaptable, reliable and accurate." It is also versatile and can be used for "sport shooting, hunting and self-defense situations," the NRA said, adding the ability to "personalize" so many of the rifle's components "is one of the things that makes it so unique."

The Brady Campaign to Prevent Gun Violence said the AR-15, a civilian model of the military's M-16, also cited the weapon's versatility in evaluating its popularity.

"They're accurate and they can basically shoot as quickly as you can pull the trigger," according to a campaign statement.

"Along those lines, they're very customizable — most average people can figure out how to install accessories like forward trigger grips that let you hold the gun at waist height and spray bullets while stabilizing the gun, laser sights, and you can add high-capacity magazines."

But Dean Hazen, owner of The Gun Experts in Mahomet, Ill., and a master firearms instructor, said the reason mass shooters are turning to the AR-15 is due to a "copy-cat" mentality more than any feature of the rifle.

"It's really just a perception thing," Hazen said. "There are rifles that are more powerful and more dangerous than that, but they're not being used."

Hazen said the AR-15 has "gotten a bad rap." He believes mass shooters generally don't know much about guns and choose the AR-15 because of the reputation it has gotten from being used in other mass shootings.

"Thank God they don't know any better because if they did they would use much more effective weapons," Hazen said.

Here is a list of mass shootings in the U.S. that featured AR-15-style rifles during the last 35 years, courtesy of the Stanford Geospatial Center and Stanford Libraries and USA TODAY research: