https://reason.com/volokh/2018/05/14/civilians-with-guns-intervening-in-activ?utm_source=quora&utm_medium=referral
Thanks to a recent FBI report, we have more data on 2016-17. The FBI found 50 shootings throughout the U.S. that it labeled "active shooter incidents"—"one or more individuals actively engaged in killing or attempting to kill people in a populated area." Of those, four involved situations where the FBI believes that "citizens possessing valid firearms permits successfully stopped the shooter." (Four others involved "unarmed citizens [who] confronted or persuaded the shooter to end the shooting.")