Keep in mind that incidents in the U.S. that had the potential to become mass shootings, such as the New Life Church shooting in Colorado in 2007, were thwarted by good people with guns in a non-”gun free zone” and were kept from becoming mass shootings.
The Crime Prevention Research Center actually already provides the most comprehensive list of those types of cases where a permitted concealed handgun has stopped these attacks.
http://crimepreventionresearchcenter.org/2015/04/uber-driver-in-chicago-stops-mass-public-shooting/