Do civilians with guns ever stop mass shootings?

"Backers of laws that let pretty much all law-abiding people carry concealed guns in public places often argue that these laws will sometimes enable people to stop mass shootings. Opponents occasionally ask: If that’s so, what examples can one give of civilians armed with guns stopping such shootings? Sometimes, I hear people asking if even one such example can be found, or saying that they haven’t heard of even one such example.

A while back I posted about a few examples, but since then there have been some more, so I thought I’d note them. Naturally, such examples will be rare. Even in states which allow concealed carry, there often aren’t people near a shooting who have a gun on them at the time. Many mass shootings happen in supposedly “gun-free” zones (such as schools, universities or private property posted with a no-guns sign), in which gun carrying isn’t allowed. And there is no central database of such examples, many of which don’t hit the national media, especially if a gunman is stopped before he shoots many victims. Moreover, at least some examples are ambiguous, because it might be unclear — as you’ll see below — whether the shooter had been planning to kill more people when he was stopped."

[Washington Post]