The Marine Corps veteran accused of killing 12 people at the Borderline Bar & Grill in Thousand Oaks, California, used a 45.-caliber Glock handgun equipped with an extended magazine. Investigators said the man legally purchased the semiautomatic weapon from a licensed dealer.

The fact that the assailant wielded a common semiautomatic pistol, the staple of the contemporary American firearms market, would seem to make this most recent gun rampage an outlier among the mass shootings that have elicited opposition to the assault-style rifles that have become closely associated with such attacks. But the numbers show that it instead makes the carnage at Jacksonville Landing routine.

The Advanced Law Enforcement Rapid Response Training (ALERRT) Center at Texas State University has studied active-shooter rampages for reports commissioned by the FBI. The FBI defines an active shooter as someone who kills or attempts to kill people in a confined or populated area. “Active shooter” is a more expansive category than mass shooting, which applies only to incidents that result in a minimum number of casualties. The term “active shooter” can apply to more targeted attacks as well as unsuccessful attempted mass shootings.

According to data compiled on 200 such attacks from 2000 through 2015, the ALERTT team found that pistols, not rifles, were the primary weapon used by the majority of active shooters:

Handguns were the most common weapon regardless of whether active shooters struck schools, businesses, or churches. The perpetrator of one of the deadliest mass shootings in history, the 2007 Virginia Tech massacre, was equipped solely with pistols. And in the 2009 Fort Hood shooting, the gunman was armed with a FN Five-seven pistol equipped with laser sights.

Semiautomatic handguns like a Glock or a Smith & Wesson M&P Shield both fire a round and load a fresh bullet with each pull of the trigger, just like AR-15 rifles. That mechanism allows a user with a quick trigger finger to discharge rounds with great speed. These handguns are fed ammunition from detachable magazines, which can be reloaded almost instantly. Like assault rifles, nearly every caliber of handgun can be fitted with an extended magazine that holds 30 or more rounds of ammunition.

What separates a rifle from a pistol is the power of the ammunition each gun fires. A rifle’s bullet is propelled at a far greater speed and carries more force. Rifles are also more accurate over long distances, whereas a pistol round is only accurate at close range. But in a confined indoor setting like the bar in California, those factors may not matter as much as the ability to quickly fire and reload.