Here's a pdf paper by John Lott et.al. covering the current state of concealed carry in the U.S. It contains all sorts of stats, charts and graphs which suggest that among other things a 1 point increase in permit holders is coinciding with a 24% overall drop in the national murder rate.
Correlation is not causality but there it is. Also as Mr. Lott admits, collecting stats from the various states is a dicey business at best as few states collect exhaustive stats on their permits, and of course there are now 10 that don't require one at all.