John Lott looks at multiple victim public shootings, and shows that the data supports conceal carry.
From the abstract:
Few events obtain the same instant worldwide news coverage as multiple victim public shootings. These crimes allow us to study the alternative methods used to kill a large number of people (e.g., shootings versus bombings), marginal deterrence and the severity of the crime, substitutability of penalties, private versus public methods of deterrence and incapacitation, and whether attacks produce “copycats.” The criminals who commit these crimes are also fairly unusual, recent evidence suggests that about half of these criminals have received a “formal diagnosis of mental illness, often schizophrenia.” Yet, economists have not studied multiple victim shootings. Using data that extends until 1999 and includes the recent public school shootings, our results are surprising and dramatic. While arrest or conviction rates and the death penalty reduce “normal” murder rates and these attacks lead to new calls from more gun control, our results find that the only policy factor to have a consistently significant influence on multiple victim public shootings is the passage of concealed handgun laws. We explain why public shootings are more sensitive than other violent crimes to concealed handguns, why the laws reduce the number of shootings and have an even greater effect on their severity.