Howard Nemerov raises an interesting point. If we take UN claims as to firearms privately owned per capita, and national homicide rates, and divide countries into quartiles based on firearms owned per capita...
The quartile with highest gun ownership had a homicide rate of 1.6. The second quartile (with under half the first's gun ownership) had a homicide rate of 2.5. Third quartile (with gun ownership a fifth of the first quartile) had a homicide rate of 2.7. Fourth quartile, with gun ownership near zero, had a homicide rate of 3.5.