The table explaining the colors is here:
File:US Violent Crime 2004.svg - Wikipedia, the free encyclopedia
The remarkable thing here is that states with relatively rural, and primarily smaller populations (e.g., the South and Desert states) have violent crime rates comparable to densely-populated urban states like California and New York. I don't necessarily attribute that totally to the prevalance of guns - a lot of it is probably just that conservative states have disproportionately violent, authoritarian cultures - but it would seem like a correlation that shouldn't be ignored.
Anyway, there's a nice factual argument for you. I still maintain that the heart of the matter is cultural rather than technical, because you and I both know that all the studies in the world wouldn't convince NRA members that gun control is a good idea - they don't care what guns do or don't do for and to the people of any given state, they just want them and insist that any level of regulation is an "infringement." Period.