These numbers are compiled by the moderators of the GunsAreCool subreddit, a sarcastically named community that tracks gun violence in America. They define "mass shooting" as any single incident in which at least four people are shot, including the gunman. The tracker comes in for some criticism because its definition is broader than the FBI's definition, which requires three or more people to be killed by gunfire. But the broader definition is nonetheless a useful one, because it captures many high-profile instances of violence — like the recent Lafayette theater shootings — that don't meet the FBI's criteria.
Some gun rights advocates — like John Lott of the Crime Prevention Research Center — object that the broader definition includes a lot of gang killings and domestic disputes that the average person wouldn't necessarily consider a "mass shooting." But there's an uncomfortable assumption here that some crime victims' lives should be valued differently — or are less worthy of attention — than others.
A more expansive definition of "mass shooting" underscores the extent to which firearms make it relatively easy to hurt large numbers of people in a very short time. With a gun, you're able to inflict bodily harm on a person once they're in your line of sight. With something like a knife or your hands, you need to get right up close to a person.
There's no easy fix to gun violence in this country. As gun rights proponents are quick to point out, municipalities with strict gun laws, like Chicago and D.C., see more than their fair share of gun crime. But it's nevertheless a fact that the level of gun violence we see in the U.S. is like nothing seen in other wealthy Western nations.