If someone commits murder in self-defense (in a position where it was impossible to do anything else).........

Do they receive any punishment? Say a guy has a gun to their head and they can't do anything but try and take the gun and kill him. If the person is successful in this endeavor, are they punished? If they're not punished, what is a customary action for the police and/or coroner to take? ....If it can be proven that it was all in self-defense and there was no way out....