"As Volokh noted, such interventions seem to be pretty rare, which is not surprising given that so many mass shootings occur in "gun-free" zones where law-abiding people are disarmed. But bystanders with firearms demonstrably can save lives, which is more than you can say for Obama's gun control proposals."

Autonomous Terrorism Calls for Autonomous Defense
Unlike Obama's gun control proposals, armed citizens can stop mass shooters who are invisible until they strike.