From the piece:
As I reported recently in our in-depth investigation, not one of 62 mass shootings in the United States over the last 30 years has been stopped this way. More broadly, attempts by armed civilians to intervene in shooting rampages are rare—and are successful even more rarely. (Two people who tried it in recent years were gravely wounded or killed.) And law enforcement overwhelmingly hates the idea of armed citizens getting involved.
Those pesky facts haven't stopped the "arm America more!" crowd from pressing the argument with alleged examples of successful armed interventions. The problem is, the few examples they keep using—in which they depict plain old folks acting heroically and with definitive results—fall apart under scrutiny. Here are five cases commonly cited and why they don't work: