I have a few friends who have dogs that have never heard a firearm being shot, yet the moment any of us are ever handling firearms in a room, the dogs will usually clear the room in a hurry. These are different dogs owned by different people.
my dog is the same way... never been exposed to gunshots but she is afraid of guns.
Another point i think many people on this thread are missing.... Why did the US Military change to .223 from a .30? Why did the US Military change from .45acp to 9x19?