5" will do the job, one lung should be popped and that deer is going down. BUT did you push him when you were looking for blood? If you pushed him, he could be a half mile away. I know it's hard not to look, but it something every hunter has to learn; if he's not down in sight, back-out, let him bed down and bleed out. I'd come back with a at least one dog of any kind and start looking. He's dead, it's just where?