As Eugene Volokh discussed earlier this month, there are indeed documented instances of "good guys with guns" stopping mass shootings. He counted at least eight cases where armed civilians (not off-duty police) appear to have prevented mass shootings since 2007. As he notes, this is probably an under-count: some would-be active shooters may have been stopped in their tracks by a brandished weapon before the body count got too high.
John Lott's Crime Prevention Research Center notes a number of other instances of armed citizens stopping would-be criminals. But some of these instances involve off-duty law enforcement officers, who are specially trained for them. Others involve armed citizens stopping knife attacks, and other altercations including armed robberies that you wouldn't normally consider a mass shooting.
On the other hand, the Violence Policy Center has documented 29 mass shootings of three or more people since 2007 where the perpetrator was himself a concealed carry permit holder. Remember Craig Hicks, the man who allegedly killed three of his neighbors earlier this year in a possible hate crime? Concealed-carry permit holder. The man who shot up DC's Navy Yard two years ago? Concealed-carry permit holder. The man who went on a rampage at the American Civic Association in Binghamton, N.Y., in 2009? Concealed-carry permit holder.
It's not necessarily the case that concealed carry permits somehow facilitated these crimes. Having a concealed carry permit doesn't make it any easier to bring, say, a shotgun into a government facility, like the Navy Yard shooter did. But these numbers might add helpful context to the idea that arming more citizens will decrease crime.
The Violence Policy Center's numbers are likely an under-count, too. After all, hundreds of mass shootings happen in this country each year. If a shooter's permit status doesn't get mentioned in media reports, we usually don't know it. But at any rate, it's clear that since 2007, among cases we can document, concealed carry permit holders have been responsible for 29 mass shootings. Eight separate mass shooting incidents were stopped by permit holders.
A few things to keep in mind. The first is that despite the media attention, mass shootings are relatively rare events -- especially compared to the tens of thousands of garden variety shootings that happen each year. As Volokh notes, the policy discussion around gun violence should be focused on reducing the larger universe of gun deaths and gun crime, not these relatively isolated incidents.
And the overwhelming majority of people who have a concealed carry permit are responsible law-abiding citizens. They will never be involved in a mass shooting in any capacity, either as a good guy or a bad guy with a gun.
But the notion that we need more armed citizens to stop mass shootings is an oversimplification. In an FBI report on active shooting incidents released last year, the agency noted that active shooters were over four times as likely to be stopped by unarmed private citizens (21 cases) as by armed ones (5 cases).
Correction: A previous version of a graphic with this story had an incorrect date for the Michael Hood shooting. It happened in March 2010, not March 2007.