More people were shot, and more killed, when the perpetrator of an “active shooting” in the US had a semiautomatic rifle, according to a new study of 248 incidents published in the Journal of the American Medical Association.

The study, which examined only a small subset of America’s shootings, found that incidents in which at least one of the perpetrator’s guns was a semiautomatic rifle left an average of 4.25 people killed and 5.48 wounded. Shootings where the perpetrator did not have a semiautomatic rifle, but instead used handguns, shotguns or other types of rifles, left an average of 2.5 people killed and three people wounded.

The type of weapon used in the shooting did not affect how likely it was that someone who was shot in an incident would die, the researchers found. Across the board, about 44% of people shot in shooter incidents died of their injuries, which came as a surprise to the study’s lead author, Adil Haider, director of the Center for Surgery and Public Health at Brigham and Women’s Hospital in Boston.

Medical workers who have treated mass shooting victims have described bullet wounds from rifles as larger and more serious than bullet wounds from handguns.

But in the study of shootings, “the proportion of people who died is actually the same”, regardless of the weapon used, Haider said. “The difference is that in incidents where there was a semiautomatic rifle, the number of victims basically doubled.”

The study, published in the Journal of the American Medical Association, also found that the vast majority of “active shooter” incidents, 75% did not involve a semiautomatic rifle.

“Active shootings” are incidents in which, according to the Federal Bureau of Investigation, one or more perpetrators “are killing or attempting to kill people in a populated area”. This government definition excludes what law enforcement officials classify as drug and gang-related shootings.

The study had some important limitations: it did not look at casualties in all mass shootings, just the subset of “active shooter” incidents as defined by the FBI. It also did not examine whether there was a difference in the fatalities or injuries in shootings that involved the military-style semiautomatic rifles classified as “assault weapons”.

In some of the incidents studied, the shooter had more than one firearm. This was the case for the majority of incidents in which the shooter had a semiautomatic rifle. But the researchers did not have data to determine which gun was actually responsible for the injuries or deaths in those shootings.

Philip Cook, an emeritus professor at Duke University and one of the America’s small number of career gun policy researchers, questioned why the researchers had focused on such a small subset of public shootings, rather than the broader category of mass shootings, which, by some definitions, occur in the US as often as once per day.

Haider, the study’s lead author, said that the researchers wanted to focus on doing as much of a “like versus like comparison” of shootings as possible. This made it better to focus on a carefully defined subset of shootings, he said, rather than looking at a variety of high-casualty shootings carried out in many different contexts or circumstances.

Cook also noted there was also a “serious error” in the paper’s brief introduction, which suggested that all semiautomatic rifles had been banned under the now-lapsed 1994 federal assault weapons ban.

In fact, Cook wrote in an email, the focus of the 1994 ban was only on on a subset of rifles with certain military-style features, “The vast majority of semi-automatic rifles sold each year were not affected,” he wrote.

Haider, the study’s lead researcher, said that what Cook described as an “error” in describing the assault weapon ban was in the short section of the paper providing background context to the readers, and did not affect the results of the study.

The average victim counts for shooter incidents was calculated excluding two incidents: the 2017 Las Vegas shooting, a “statistical outlier” that left 58 people killed and and 489 people wounded, and the 2015 San Bernardino attack, in which there were two shooters.