How many times in real life has an armed intruder, bent on mass murder, been taken out by an armed civilian (or even a police officer) before he could kill anybody?

I'll rephrase that. How many times does it even *appear* that a hero has prevented a mass killing by drawing down faster than the heat-packin' bad guy?

Or even managed to *cut short* a shooting spree by nailing the gunman when he wasn't looking?

Come on, guys. How many?

I want to know.