In the best published study of mass shootings, it was found that 99 had occurred between 1980 and 2010. We know that a number of mass shootings have been stopped by armed citizens. All but one of the “successful” mass shootings occurred in areas where armed citizens are banned. A comparison of mass shootings stopped by 911 response vs mass shootings stopped by citizens indicates that when citizens are able to stop the killing, they prevent 84 percent of the deaths.
Definitions and precise numbers vary by researcher, but it is clear that at least 10 percent of mass shootings are stopped by armed citizens. I believe this is the minimum number, the actual number may be much higher, because when a citizen stops an intended mass shooting early, it never becomes a successful mass shooting and may never become a news story of note. For example, in the recent Clackamas Mall shooting, an armed citizen may well have cut the killings short without firing a shot. While we can never be certain, a large percentage of mass shooters stop killing and commit suicide when they are confronted with armed force.
The problem of quantifying mass shootings stopped by armed citizens is exacerbated by the inablility to know the intentions of a dead attacker. Some attackers leave notes, making their intentions clear, as did the Santa Clara shooter in 1999, or the AT&T store shooter in 2010. Some announce their intentions, or otherwise indicate that they intend to kill many people. But these are the rare cases. If an attacker attempts to kill many, but is stopped at the beginning of the attack or after a few shots are fired, it is impossible to know if a mass killing was stopped, or if it was only one of thousands of more ordinary crimes stopped by armed citizens.
It is clear that the number of mass shootings has decreased as the number of armed citizens has increased. Three decades ago, there were very few citizens with concealed carry permits. Most states did not allow concealed carry. The increase in concealed carry states mostly occurred after 1994, and the bulk of concealed carry permits have been issued after the turn of the new millenium. Even today, less than 2.6% of the population has concealed carry permits, and 30 percent of the population of the country live in states where it is very difficult to obtain a permit, or in the case of Illinois, impossible. The first decade of the millennia showed a remarkable drop in mass shootings, a 44 percent reduction, according to Grant Duwe, a criminologist with the Minnesota Department of Corrections . These numbers are compatible with research published in 1999, that showed a drop in mass shootings with increased concealed carry permits.
While it is too early in this decade to determine if the trend will continue, reducing the number of places that ban armed citizens will likely result in more citizens being able to stop the mass shootings in the early stages, sometimes before any innocents are killed.