Active shooters are stopped by armed civilians in 3% of cases, according to the most recent FBI study. Unarmed civilians stop the active shooter in 13% of cases. (http://www.fbi.gov/news/stories/2014/sep...n-2000-and-2013).

NYPD's research concludes that shooting an assailant is the most effective countermeasure in stopping an active shooter (Active Shooter: Recommendations and Analysis for Risk Mitigation, NYPD, 2012; http://www.nypdshield.org/public/SiteFiles/documents/Activeshooter.pdf)

A Purdue professor's research shows that all but two active shooter events since the 1950's occurred somewhere that concealed carry was illegal (http://www.theblaze.com/stories/2014/04/...uns-in-schools/). Only one active shooter incident in recent years occurred somewhere that concealed carry was legal (Rep. Giffords) (and in this case, a concealed carrier came upon the scene immediately after the shooter was subdued by unarmed bystanders).

Deaths and injuries from mass public shootings fall dramatically after right-to-carry legislation is passed. Multiple-victim shootings decrease in states that have concealed carry (Multiple Victim Public Shootings, Bombings, and Right-to-Carry Concealed Handgun Laws: Contrasting Private and Public Law Enforcement, Lott John R., Landes William M.; University of Chicago – covers years 1977 to 1995).