A good case could be made that the more people that have guns the less likely mass shootings are to occur. It a little tough to do the experiment because how do you know of a mass shooting was stopped because more people have guns?
But there is evidence even if we can’t do a fully controlled experiment like we would like. There are a number of cases listed here (H/T to John Lott) where the bad guy left a note or had made a modest start toward challenging previous mass shooting body counts when they were stopped by someone with a CCW and a gun.
There is also evidence presented in John Lott’s book, More Guns, Less Crime as well. In the first edition (not sure about later revisions) the number of mass shootings prior to allowing people to exercise their right to keep and bear arms in public (no CCW) there were a modest number of mass shootings, then after passage of right to carry laws the mass shootings in that political jurisdiction went to zero. This was prior to the Gabby Giffords shooting which, of course, was in the gun friendly state of Arizona. But given that one exception when mass shooting occurs you can make a very safe bet that it happened in a “Gun Free Zone”.
It’s time to stop the mass shootings. We must stop government from infringing on our rights.