The availability of guns (or, more specifically, firearms) has been measured in numerous ways, including the % of suicides committed with firearms, the % of homicides committed with firearms, and the "Cook index" (the average of the % of homicides committed with firearms and the % of suicides committed with firearms), and the number of officially issued firearm licenses.
Association with rates of violence
Gun availability is associated with higher homicide rates, as well as higher suicide rates and higher accidental gun death rates. The availability of illegal guns, but not that of legal guns, is associated with higher rates of violent crime.
References
Gun availability Wikipedia(Text) CC BY-SA