A gun is a tool, like a hammer or a drill. Some tools are more efficient at their jobs than other tools. For instance, a cordless electric drill allows the user to drill more holes or screw in more screws than an old-fashioned hand-crank drill. The purpose of a gun, broadly speaking, is to kill or wound, and a more efficient gun, one that can fire more rounds more quickly, can kill or wound more efficiently. Based on this logic, it makes perfect sense that in the event of mass shootings in the United States, ones involving semiautomatic rifles typically produce more casualties than ones in which the shooter used less-efficient weapons.

As it turns out, mass shootings where semiautomatic rifles were used are 71 percent more deadly than when only non-semi-automatic rifles are used, according research into 17 years’ worth of mass shootings.

In a research letter published Tuesday in the Journal of the American Medical Association, a team of doctors and public health researchers at Brigham and Women’s Hospital in Boston combed through the years of FBI data on mass shootings and found that shooters who used a semiautomatic rifle — regardless of whether other weapons were also present — wounded almost twice as many people, and killed almost twice as many people, compared to shootings in which the perpetrator did not have a semiautomatic rifle.

Gun rights rally "come and take it" flag
Gun owners gathered at the Minnesota Capitol to protest against new gun control laws, including restrictions on semiautomatic rifles.

Based on the FBI database, there were 250 active shooter events, defined as “a situation in which an individual is actively engaged in killing or attempting to kill people in a confined or populated area,” between 2000 and 2017. The 2017 Las Vegas shooting that involved a bumpstock-equipped rifle fired into a crowd was excluded as a statistical outlier for its 59 deaths. The 2015 San Bernardino shooting was excluded because it involved two shooters.

Why It’s Hard to Study Mass Shootings as a Public Health Matter

From the remaining 248 shootings, researchers found that 898 people had been shot and 718 had been killed. They scoured news reports (three for each shooting) to find out whether the perpetrators had been armed with semiautomatic rifles. They had to do this because FBI data, despite being the only comprehensive source available, do not indicate what kinds of rifles shooters used. This paucity of data is due, in large part, to the 1996 Dickey Amendment, which stipulates that the US Centers for Disease Control and Prevention may not use federal funds to study gun violence. Because of this bill, federally funded public health officials have been hamstrung in their ability to study gun violence as a public health issue.

This video released in June 2016 shows a Ramsey County, Minnesota Sheriff’s Office deputy installing a cable lock in a semiautomatic rifle. Proper safety procedures can reduce the risk of accidents, but semiautomatic rifles still possess deadlier potential than other classes of firearms.

What Guns are Used in Mass Shootings?

Out of the 248 shootings over 17 years tracked by the FBI, 76 involved a rifle, and in 61 of those cases, there was a semiautomatic rifle. A semiautomatic rifle can fire a single round with each trigger pull without the need to reload between shots. This feature allows a shooter to fire a semiautomatic rifle as fast as he — almost all mass shooters have been men — can wiggle his finger, unhindered by the need to reload.

In the case of a shooter who’s using an AR-15 semiautomatic rifle, the extremely popular weapon that was used in the Pulse nightclub shooting and the Marjorie Stoneman Douglas High School shooting, he can let off up to 45 rounds per minute.

2018.06.12 A Candlelight Vigil to Remember Pulse, Washington, DC USA 03771
A shooter at the Pulse nightclub in Orlando killed 49 people with a semiautomatic rifle.

97 Percent More Deadly

With this kind of firing capacity, the study’s authors found, an average of 4.25 people died in each shooting involving a semiautomatic rifle compared to 2.49 people who died in shootings in which a semiautomatic rifle was not used, a difference of 71 percent. But when the researchers ran a negative binomial regression that adjusted for the place of shooting, year of shooting, and presence of other firearms, this difference ballooned to 97 percent. This difference is shown in the dark blue bars in the chart below.

graph on semiautomatic weapons used in mass shootings
This chart shows data on semiautomatic shooter incidents versus non-semiautomatic incidents.

They also observed that an average of 5.48 people were wounded in shootings involving semiautomatic rifles versus 3.02 in shootings without them. While the researchers found that about 44 percent of mass shooting victims died from their wounds regardless of what kind of weapon was used, more people were injured and killed when the shooter used a gun that could shoot more quickly.

“Semiautomatic rifles are designed for easy use, can accept large magazines, and fire high-velocity bullets, enabling active shooters to wound and kill more people per incident,” the study’s authors write.

Notably, the time period encompassed by this dataset straddles the expiration date of the federal assault weapons ban of 1994. This law expired in 2004 under President George W. Bush, making it legal to own and possess weapons that had been designated “assault weapons” under the law. These included semiautomatic rifles with certain features, including pistol grips and high-capacity magazines — features that the ubiquitous AR-15 possesses. It excluded some semiautomatic rifles, though, such as those with normal rifle grips and small magazines. The federal assault weapons ban made large-capacity magazines — those that hold more than 10 rounds — illegal, but now these items are bought and sold openly. The study did not, however, indicate whether the number of shootings involving semiautomatic rifles went up after the ban expired.

In addition to the concerning statistics on mass shootings, this study highlights the alarming lack of data on mass shootings in the United States.

“There’s just no support for gun research,” Adil Haider, M.D., M.P.H., a trauma surgeon and public health researcher at Brigham and Women’s Hospital and the study’s lead author, tells Forbes. “That’s why you haven’t seen a lot of this, and even these basic questions have not been answered.”