On TV a while back, they had the old men who were trained to be guerrillas in case the Germans invaded England.
What they were taught for sentries was to come from behind, yank the head back and let the edge of the helmet break the neck.
Seemed it worked with German helmets, not sure if it would work with today’s helmets though.
So, what is the prescribed response to having someone grab your mouth and nose from behind, while you are on sentry duty?