Recently, though, some have claimed that the law reduced mass shootings in particular, which account for a tiny fraction of overall homicides but command an incredibly disproportionate amount of public attention. What seems to be true is that the ban years were relatively peaceful on this front, despite covering the rash of school shootings that included Columbine. They were especially peaceful compared with the past ten years or so, which have seen an alarming rise in this form of terrorism.
However, the three most important facts about mass shootings are (A) they have historically been incredibly rare, with entire years passing without one sometimes; (B) they are contagious, with high-profile incidents inpiring copycats and competitors; and (C) they are incredibly variable in the number of fatalities, from a low bound of wherever the researcher chooses to set it (the study Biden cites uses four, not including the perpetrator) all the way up to 58 at Las Vegas. Trying to detect a pattern in data like this, and then attributing the pattern to a single law change that covered the entire country for a ten-year period, is madness.