Ready access to guns, also ready access to alcohol, ready access to knives, etc these are all part of the equation, but I contend that they are so dwarfed by the other factors that focusing on them seems disingenuous and otherwise random except as a result of knee-jerk politics.
So maybe cities have the same order of magnitude effect in increasing crime as guns do - what would you suggest we do about that? Should we lower violent crime by banning cities and banning guns, or should we focus on the root problems of culture, social cohesion, and poverty?
Also: Switzerland also has a ton of guns and major cities, violent crime rate is near the bottom - is suspect because their culture, social cohesion, and efforts to minimize poverty.