Louis Klarevas is a research professor at Columbia University Teachers College and the author of Rampage Nation, which examines American mass shootings and efforts to prevent the killings. His research looked at mass shootings resulting in six or more deaths, which he refers to as massacres, because they are “the most dangerous and threatening to American public safety.”

He found that deaths in such high-fatality mass shootings dropped by 25 percent under the ban. Massacre deaths involving assault weapons fell by 40 percent, fatalities involving both assault weapons and high-capacity magazines fell by 54 percent. After the ban lapsed, mass shootings and related deaths surged. “We know that during the federal assault weapon ban, we saw some significant reductions in high-fatality mass shootings compared to the decade before the ban and the decade after it expired,” he said. “What we don’t know exactly is the mechanism behind that change.”

A 2019 study looking at the impact of the federal law on mass shooting deaths arrived at a similar conclusion: While new assault weapons and high-capacity magazines were outlawed, mass shooting fatalities were 70 percent less likely than during the decades before and since.
https://www.thetrace.org/2019/08/what-the-data-says-asssault-weapons-bans/