Backers of laws that let pretty much all law-abiding 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 even one such example. Naturally, such examples will be rare, partly because mass shootings are rare, partly because many mass shootings happen in supposedly “gun-free” zones (such as schools, universities, or private property posted...