From Volokh Conspiracy:

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.

1. In Chicago earlier this year, an Uber driver with a concealed-carry permit “shot and wounded a gunman [Everardo Custodio] who opened fire on a crowd of people.”

2. In a Philadelphia barber shop earlier this year, Warren Edwards “opened fire on customers and barbers” after an argument. Another man with a concealed-carry permit then shot the shooter; of course it’s impossible to tell whether the shooter would have kept killing if he hadn’t been stopped, but a police captain was quoted as saying that, “I guess he [the man who shot the shooter] saved a lot of people in there.”