Law professor Eugene Volokh
considers the question posed above and discusses four instances that he's immediately aware of in which an armed civilian was able to prevent or minimize a mass murder. Volokh writes:
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 with a no-guns sign) in which gun carrying isn’t allowed, and partly for other reasons. Moreover, at least some examples are contested, because it might be unclear ... whether the shooter had been planning to kill more people when he was stopped. But here are instances that I have seen, not counting killings stopped by people who were off-duty police officers (or police officers from other jurisdictions) at the time of the shooting.
You can read about his examples at the link. Meanwhile,
this chart shows some interesting data from 2010. In cases in which the murder weapon was known, slayings committed with the aid of rifles, including semi-automatic weapons such as were used in the Sandy Hook massacre, were a small fraction of the murders in this country. And, it was a bit of a surprise to learn, twice as many people were murdered with fists and feet than with shotguns or semi-automatic rifles:
