This is a study for listopencil, since he was asking for research that excluded suicides:
This paper provides new estimates of the effect of household gun prevalence on homicide rates, and infers the marginal external cost of handgun ownership. The estimates utilize a superior proxy for gun prevalence, the percentage of suicides committed with a gun, which we validate. Using county- and state-level panels for 20 years, we estimate the elasticity of homicide with respect to gun prevalence as between +0.1 and + 0.3. All of the effect of gun prevalence is on gun homicide rates.
Higher gun prevalence = higher homicide rates.