What if there are more guns because people in those areas feel they need to protect themselves? It's hard to show which came first. Or maybe it isn't and someone has already done that.
A good question that can be tricky to answer. Per the abstract:
The association between firearm prevalence and homicide victimization in our study was driven by gun-related homicide victimization rates; non-gun-related victimization rates were not significantly associated with rates of firearm ownership. Although causal inference is not warranted on the basis of the present study alone, our findings suggest that the household may be an important source of firearms used to kill men, women and children in the United States.
* higher non-gun murder rates did not correlate to higher gun ownership (people didn't find the need to buy guns to protect against non-gun crime)
* still can't draw an explicit cause-effect relationship (causal inference is hard)