Ahh good old J. Edger's Uniform Crime Report (his greatest contribution to Law Enforcement). Unfortunately they are limited by the fact that they do not tabulate crimes by whether or not the area allows firearms to be carried, but instead by geographic region. I imagine that this information could be combined with some NIBRs (incident based reporting) and victim impact surveys to create a well rounded and compelling sets of data and facts. As far as I've found though there doesn't seem to be a study of this kind, but perhaps I'm mistaken (it has happened before )