Europe,
Is the variation in the group after 5 shots because of the ammo variation or because of the temperature change in the barrel?
If it is the ammo, are the fellows shooting .5 moa using hand loads that are just a little more consistent and matched to their rifles?
Without a rear bag, I'd have trouble shooting under 1.5 moa at 100 meters with a perfect rifle and perfect ammo, I think. Getting a steady hold would be much more significant than the barrel heat, for example.