From The American Spectator:

Homicide in America is far more common than it ought to be. But mass shootings — defined as four or more murders in the same incident — constitute a minuscule share of the total, as I discuss in “The Shooting Cycle” in the most recent edition of the Connecticut Law Review

The Bureau of Justice Statistics reported that from 2002-2011, 95 percent of total homicide incidents involved a single fatality, 4 percent involved two victims, 0.6 percent involved 3 victims, and only .02 percent involved four or more victims. Another study performed between 1976 and 2005 yields similar results — that less than one-fifth of 1 percent all murders in the United States involved four or more victims. In other words, the bottom line is that out of every 10,000 incidents of homicide, roughly two are mass killings.