"The nation's mass-shooting problem seems to be getting worse. And the latest, most serious shootings all seem to have one new thing in common: the AR-15 semi-automatic assault rifle. The AR-15 typically has large magazines, shoots rounds at higher velocities than handguns, and leaves more complex wounds in victims. In each one of the older shootings on the 10-deadliest list — including the 2007 attack on Virginia Tech in Blacksburg, Va., that left 32 victims dead — the shooters carried handguns. (The exception is the 1984 San Ysidro massacre, where the gunman also used a shotgun and an Uzi semiautomatic carbine.) But in all of the latest incidents — Newtown, Conn., in 2012; San Bernardino, Calif., in 2015; Orlando, Fla., in 2016; Las Vegas, 2017; Sutherland Springs, Texas, 2017 — the attackers primarily used AR-15 semiautomatic rifles. There are a couple of theories that might suggest why AR-15s would be associated with deadlier attacks. AR-15 rifles shoot small but high-velocity .223-caliber rounds that often shatter inside victims' bodies, creating more devastating injuries than the wounds typically left by larger but lower-velocity handgun rounds. Shooters also commonly use the rifles with 30-round magazines, which allow them to fire more rounds uninterrupted, compared with the smaller magazines commonly used in handguns."