Man, I don't know about the smallest, never hunted a gorilla before. I bet a .45 would still stop him. You hit him in the heart a 9mm would do the trick probably. Actually, I bet with just about any handgun you'd better hit him in the heart. Unless you got him in the eye or mouth I don't know about penetrating the skull of a gorilla. I think it's pretty thick and tough. Any primate experts out there?