The thing of it is, there's not many records of civilians stopping mass shootings with a gun, because those cases never had a chance to reach mass shooting status. A guy walks into a crowded area and starts shooting and kills 15, it makes national news, calls for gun control, etc.
Now take a guy walks into that same crowded area, starts shooting, and injures one before being fatally shot by an armed civilian. At best it makes local news. Every time an attacker is shot by an armed civilian, it could be the stopping of a potential mass shooting. But it'll never make waves like the successful attacks will. That's the problem that again goes back to media. It doesn't wanna show the news where there was no carnage, as that won't get ratings, and won't push agendas. But the news of a successful mass shooting? That will.
I can't add much to the debate because you and 1UP have got it covered perfectly, but I would like to share my two cents worth. First and foremost, I've always been around firearms. From rifles to machine guns and other NFA regulated firearms and/or devices. But what I enjoy watching are the arm citizen/civilian videos. Where individual(s) are up to no good, typically armed robberies with multiple suspects, or an individual targeting a gun-free zone with the intention of killing a lot of people. But are stopped by someone who is armed and there is little to no loss of life minus the perpetrator.
There are literally thousands of videos that corroborate the fact that armed citizens are the first line of defense in shootings, robberies, muggings, just about all major crime. I just cringe at the fact that people think relying on the police should be the first line of defense. I've done my fair share of ride alongs and we responded to a call code 3 and it still took us almost 8 minutes to get to where we were going. Hell we had a deputy who was killed last month and the first unit on the scene - who obviously responded code 3 - didn't get there soon enough. I believe the idiom is, "If seconds count, law enforcement is only minutes away." Or something along those lines.