The Crime Prevention research center, and John Lott, runs a lot of those numbers. But you can find them in many places.

It's not just homicide rates that have gone down---it's all categories of violent crimes. And yes, we're talking per capita rates. The argument that medical care has prevented what otherwise would have been higher murder rates (an argument that Col. Dave Grossman makes all the time) doesn't stand up to the rest of the data on other violent crimes, like robbery, shootings, assault, rape, etc, whose rates have all gone down dramatically. Non-fatal violent crime rates have been sharply declining, and so has even property crime rates. This can all be found in the National Crime Victimization Survey (the gold standard poll for criminology numbers) and the FBI and US Justice department.

There are a lot of facts that are hard to believe---and actually looking at the empirical data can be very illuminating.