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.
Still, for whatever they are worth, here is a list of some such incidents (which deliberately excludes killings stopped by people who were off-duty police officers, or police officers from other jurisdictions, at the time of a shooting, as well as some other cases which struck me as borderline):