Handguns are almost three times as likely to be used in mass shootings as rifles, according to a new report from a gun-safety group that appears to cut against gun-control advocates’ push to ban some semiautomatic rifles.

More than three quarters of mass shootings from 1996 to 2016 involved a handgun, while just 29 percent of shooters used a rifle, according to the New York-based Rockefeller Institute, part of a gun safety initiative convened by a handful of mostly Democratic governors.

Some shooters, such as the attack at a Texas school earlier this month, carried both a handgun and a long gun.

The relatively small percentage of mass shootings involving rifles busts one of the “myths” of the gun debate, the new study said.

“I think it would come as a surprise to a lot of people,” said Robert J. Spitzer, a political science professor at SUNY Cortland who is part of the research arm of the multi-state gun group though he didn’t write this new report.

He added, though, that the use of rifles is on the rise in high-profile mass shootings, and said they often account for a higher death toll in shootings.

For example, last year’s Las Vegas shooter “would not have been able to do what he did with a couple of handguns,” Mr. Spitzer said.

Police say Stephen Paddock killed 58 people and injured hundreds of others when he opened fire from his hotel window down onto a music festival. Authorities found in Paddock’s hotel room several “bump stock” devices used to convert semiautomatic weapons into automatic-style ones.

“So for those kinds of reasons, there is a legitimate public policy reason to focus on assault weapons, but it’s also important not to take your eye off the fact that handguns are indeed more often used,” Mr. Spitzer said.

Second Amendment supporters said the findings should derail efforts to enact stricter laws, such as a ban on certain semiautomatic rifles, in the wake of the last year’s worth of high-profile massacres.

“Given that the left is on a jihad to demonize the fictional ‘assault weapon,’ it was refreshing to see the Rockefeller report set the record straight on handguns,” said Erich Pratt, executive director of Gun Owners of America.

The report was the first one from a regional gun violence research consortium organized earlier this year by the governors of New York, Connecticut, New Jersey, Massachusetts, Rhode Island, Delaware, and Puerto Rico.

The governors’ offices did not answer questions about the report Tuesday, though New York Gov. Andrew Cuomo did provide a recent link to it on social media, saying the group is “supporting critical research to better understand mass shootings and how to prevent them.”

Connecticut Gov. Dannel Malloy said more research on the subject is “desperately needed” and that he looked forward to reviewing the report.

He said Republicans in Congress “folded to the will of the NRA” by routinely including language in spending bills that’s been interpreted as an effective ban on government research into gun violence.

“By working together with like-minded states, we can take strides toward understanding the root causes of violence and determine the most effective strategies to prevent all forms of gun violence — not just mass shootings,” Mr. Malloy said.

The study’s authors cast it as an initial effort to compile basic information on mass shootings: how often they happen, who commits them, where they’re committed, and trends over the past 50 years.

The authors defined “mass shooting” as an incidence of “targeted violence” carried out at a public place that resulted in multiple victims, occurred within a 24-hour period, and wasn’t gang-related.

But they also acknowledged that different groups use different definitions for what they consider to be mass shootings, and said that lack of uniformity is an obstacle to producing reliable research on the subject.

The study found that mass shootings have steadily increased over the past 50 years, but are still “statistically rare.” The likelihood of being injured or killed in a mass shooting was about seven times greater between 2006 and 2016 than it was 40 years earlier — though the “average incidence rate” was still about 0.04 per 100,000 people from 2006-2016.

Of the 340 mass shootings the report chronicled over the past 50 years, 57 percent took place either at a workplace or a school.

Nearly all of the shooters — 96 percent — were male, and were 33.4 years old on average. Fifty-four percent of the shooters were white, compared to 27 percent who were black, 9 percent who were Hispanic, and 4 percent who were Asian.