Could we please dispense with this nonsense? When was the last time you heard of this happening? Never?
From a piece in Mother Jones (my emphasis):
As many commentators noted, it was particularly callous of the NRA to double down on its long-standing proposal to fight gun violence with more guns while parents in Newtown were burying their first graders. But more importantly, the NRA's argument is bereft of supporting evidence. A closer look reveals that their case for arming Americans against mass shooters is nothing more than a cynical ideological talking point—one dressed up in appeals to heroism and the defense of constitutional freedom, and wholly reliant on misdirection and half truths. If only Sandy Hook's principal had been packing heat, the argument goes, she could've stopped the mass killer. There's just one little problem with this: Not a single one of the 62 mass shootings we studied in our investigation has been stopped this way—even as the nation has been flooded with millions of additional firearms and a barrage of recent laws has made it easier than ever for ordinary citizens to carry them in public places, including bars, parks, and schools.
Attempts by armed citizens to stop shooters are rare. At least two such attempts in recent years ended badly, with the would-be good guys gravely wounded or killed. Meanwhile, the five cases most commonly cited as instances of regular folks stopping massacres fall apart under scrutiny: Either they didn't involve ordinary citizens taking action—those who intervened were actually cops, trained security officers, or military personnel—or the citizens took action after the shooting rampages appeared to have already ended. (Or in some cases, both.)