n the wake of the unthinkable massacre in Connecticut, pro-gun ideologues are once again calling for ordinary citizens to arm themselves as a solution to mass shootings. If only the principal at Sandy Hook Elementary School had possessed a M-4 assault rifle she could've stopped the killer, they say. This latest twist on a long-running argument isn't just absurd on its face; there is no evidence to support it. As I reported recently in our in-depth investigation, not one of the 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.