With the recent Connecticut school shooting, the debate is on with regard to whether the solution is more guns or fewer guns. One of the issues in that debate is whether legitimate firearms in the hands of private citizens would reduce the number of deaths by stopping mass shootings earlier. I wasn't able to find any peer reviewed articles on the issue, but I did find an analysis that is of comparable quality.
The analysis, of 100 mass shootings, looked at those that were stopped before they were voluntarily ended by the shooter. Out of 100 incidents, 15 were stopped by police arrival or action with an average of 14.29 killed per incident. 6 were stopped by armed civilians with an average of 1.8 killed per incident. Looks like a strong argument in favor of the "more guns" argument, right?
But wait! There were also 11 incidents stopped by unarmed civilians, with an average of 2.6 killed per incident - nearly as low as the ones stopped by armed civilians. If guns help - which they appear to - they may help more by increasing the frequency with which a civilian bystander feels empowered to stop the incident than by reducing the number of casualties relative to situations where civilians stop the incident without a gun.
In any event, the most important factor is whether civilian bystanders act to stop the incident, whether or not they have a gun.
Analysis, including a summary of all 32 incidents stopped by police or civilians: