I haven't ever been in combat of course but in almost any stressful situation, you are going to see a drop off compared a leisurely pace. What makes the biathlon so difficult is not just the skiing and not just the shooting, it is trying to shoot after you cross-country ski when your heart rate is way up.
The targets are 50 meters/164 feet in American away and are either 115mm/4.5" standing or 45mm/1.77" when shooting prone. On my best day in perfect weather conditions those are not easy shots. In the winter after you are racing on cross country skis and your heart is pounding? Yeah, I am not hitting any of the targets, mostly because I died of a heart attack on the track.
The point being that in a stress situation where your heart is racing and you are hopped up on adrenaline, it isn't like shooting from a casual pose on the range with your feet set and the ideal grip. As the saying goes, train like you fight and these are pretty good recommendations.