As of this week, seven of the 10 deadliest mass shootings in modern U.S. history have all happened since 2007. And the latest, most serious shootings seem to have one new thing in common: the AR-15 semiautomatic assault rifle.

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When a gunman identified as Nikolas Cruz attacked a high school in Parkland, Florida, on Wednesday, killing at least 17 people, the event marked a depressingly familiar milestone.

As of this week, seven of the 10 deadliest mass shootings in modern U.S. history have all happened since 2007. And the latest, most serious shootings seem to have one new thing in common: the AR-15 semiautomatic assault rifle. Indeed, police said Cruz was armed with at least one AR-15 semi-automatic rifle and “countless magazines.”

In each one of the older shootings on the nation’s 10-deadliest list, with one exception, the shooters carried handguns. But in all of the latest incidents — Newtown, Connecticut, in 2012; San Bernardino, California, in 2015; Orlando, Florida, in 2016; Las Vegas in 2017; and Sutherland Springs, Texas, 2017 — the attackers primarily used AR-15s.

There are some theories that might suggest why AR-15s would be associated with deadlier attacks.

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AR-15 rifles shoot small but high-velocity .223-caliber rounds that often shatter inside victims’ bodies, creating more devastating injuries than wounds typically left by larger but lower-velocity handgun rounds.

Shooters also commonly use the rifles with 30-round magazines, which allow a shooter to fire more rounds uninterrupted.