This article gives some hard numbers about murder rates.
It's good to have some facts on tap, but keep in mind that the 2nd Amendment isn't about reducing the murder rate. To defend the 2nd Amendment on those grounds is to give legitimacy to the claim that it's okay to disarm the people if only they can be promised security. In fact the Founders gave us the 2nd Amendment as one of the principle means of limiting the power of government, and keeping power in the hands of the people. Overwhelming power inevitably degenerates into tyranny, no matter how well-intentioned it began. The Communists and Nazis and Fascists of the 1930's sincerely believed theirs was a glorious cause. It's sad and tragic that the same group that, in the 60's was chanting “power to the people” are today the advocates of authoritarian rule.