https://www.sciencedaily.com/releases/2018/05/180531171607.htm

Studies have shown that these laws, which are sometimes called permit-to-purchase licensing laws, are associated with fewer firearm homicides at the state level. This is the first study to measure the impact of licensing laws on firearm homicides in large, urban counties, where close to two-thirds of all gun deaths in the U.S. occur.

The study was published online May 22 in the Journal of Urban Health and was written by researchers at the Johns Hopkins Center for Gun Policy and Research, based at the Johns Hopkins Bloomberg School of Public Health, and the Violence Prevention Research Program at the University of California, Davis.