I wouldn't, and will not, shoot at rocks. Also, I would be carrying a revolver with LEAD bullets, which squash on hard surfaces rather than ricochet or shatter (although the rock might shatter). I would in all probability load it with blanks. I don't want to kill anything, just frighten it off. The only weapon I can think of that would probably drop a bear around here is a 12 gauge(bore)shotgun shooting solid slugs...and even then it's iffy.

Bear spray is better, assuming that, like firearms, you know how to use it.