The media is pretty quick on reporting mass shootings. But you don’t hear of when mass shootings are avoided, especially when a gun is used to end the mass shooting before it begins.
This Ain’t Hell (one of my favorite blogs) has a story of a partygoer who was asked to leave the party. He comes back with a gun, presumably to shoot up the place. Only one problem. A different partygoer with his own firearm. The end result is the mass shooter was shot before he could shoot anyone else. AZ Central has the details.