A slug bouncing off bone is entirely possible. I shot a small doe standing broadside high from about 35yds and it bounced off her spine and back out the same side. By the time it exited it had expanded and left a hole big enough to pull her heart out of. Look to the top left of the exit and you'll see a round circle about as big as your thumb, that's the entrance. Both the entrance and exit were confirmed when she was skinned and gutted, you could see the point where the slug hit the spine.
