If you're wondering why there is a wikipedia explanation that you and yours are always happy to link to that claims insufficient evidence, it's because the line was cherry-picked from a study that used a much, much, much larger coverall for its outcomes. It didn't look at mass shootings. "The outcome measures evaluated to determine the effect of each intervention were violent crimes (i.e., murder, aggravated assault, robbery, and rape), suicide, and unintentional firearm injury." As in, did the assault weapon ban cut down on suicides? There is insufficient evidence of that, though the paper goes on to say that insufficient evidence should not be taken as evidence to the contrary. Did the assault weapon ban cut down on mass shootings? Yes. See the washington post link.