I agree that fear of not being believed, and stigma, those too might prevent a rape from being reported. "Is there evidence that more guns lead to less crime?" I do not know for certain, if there is evidence I'm not sure that it would be uniform world-wide.

As far as america is concerned someone actually wrote a book with that exact title and makes his argument that yes indeed violent crime rates go down in states with concealed carry laws. So maybe it's not more guns per se.

Just the wild card factor that anybody could be packing, that would prevent people criminals from randomly picking marks because s/he thinks the person is not armed.