I'd need more context, I guess. If the cop is off duty, it really isn't a police department matter. He's a civilian, and could be charged criminally with assault in the second or third degree, driving while intoxicated, and could be held civilly liable for money damages for the victim's injuries. Without any other context, I think what the court is saying is that it is not an administrative tribunal, and that, indeed, is so. The fact that the gun might have been his department gun is relevant administratively, but not criminally, unless there is a specific statute in that state saying otherwise.