Oh, as a note, I'm only writing about this because the question is ticking me off. My thoughts on gun control are my own, and probably would surprise you. My thoughts on media (either side) manipulation are another thing, and that is what this is about.
So, let's begin.
The left wants all guns outlawed, or guns with pointy bits, or background checks, or whatever they want, the right wants more people, babies, woodland creatures armed with guns to prevent these lone psychopaths. Each solution is the diametric opposite of the other, and neither is right in my opinion, but that's not what I'm writing about.
Right now the current left statement is that no mass shooting has ever been stopped by a lone armed civilian. This is correct. Keep reading though.
Gun advocates will also say that plenty of mass shootings have been stopped by civilian armed. This is also correct.
There's a statistic floating around by the gun side saying average mass shooting stopped by cops: 14+something dead. Average mass shooting stopped by individuals carrying firearms 2.7. This is technically a lie, but it is the truth as far as I can tell.
How can all of that that be? Well here's the thing. A mass shooting involves 4 or more people being shot and (presumably) killed. If you're looking for incidents in which mass shootings have been stopped, this means that 4 people had to be shot, and they had to be shot in one set of shootings, not over multiple instances.
If a potential killer loads up 40000 rounds of ammo and goes to an event where he knows very few people will be armed, a civilian with a firearm sees this and engages in a firefight and stops or kills the person who would have murdered hundreds with nobody else killed or shot, the armed civilian has not stopped a mass shooting, the armed civilian has stopped a potential shooter. The would-be mass-murderer wasn't, and so the statistic that a civilian stopped a mass murderer or made a difference never changes.
If the armed civilian were aiming at changing that statistic, he could sit out and wait until 4 people were laying dead and then bring the murderer down. Only at that point would an armed civilian have stopped a mass shooting/murderer. By definition.
This is where the problem is and this is why so many people thing the other side is lying when they're not. Yesterday, it took me twenty seconds to find two armed civilian stories that involved them stopping the rape and murder of over 4 people. There was also another story, which appeared a bit biased, in which an armed gunman intent on killing as many as possible was brought down with only one death other than his own. In both instances, by definition, the armed civilian has not stopped a mass shooter or a mass murderer... he has however stopped a potential mass murder.
So yeah, right and left media are lying and the consumers of this media are being fed biased misinformation. The question to look at is: Are armed civilians stopping potential mass murderers - in which case you'll need to be a bit subjective and determine if the person stopped possessed the capacity to kill a lot of people.
And I'm not going to try and answer that, I'm just saying that's the question. Anything else is stupid.
So yeah, get your data straight, then argue one way or the other. Don't throw broken statistics. Once your argument is there, get back to it.