Ouch! I don't know if it was a BB gun that caused it, but I really hope not! The stray that I take care of ended up with a similar wound in the same exact place - base of the tail - last summer, though if I remember, it was more of a gash than a hole. He seemed fine otherwise, so I kept an eye on him. After a couple of days, he started licking furiously, so I knew it was starting to heal. I never figured out what it was. It was too deep to be from a fight with another cat. I thought he might have tried to slink under a metal fence and gotten cut maybe. But it did heal completely, and he's absolutely fine. I hope it's the same for your kitty.