I've had several hunts where the temp was between 0F and -25F. I could hold out for an hour and a bit but by then I was too cold to shoot. I've got excellent snow packs and don't have cold feet, a good balaclava to keep my head warm and keep my hands in my handwarmer pockets until I need to shoot. So my main concern is keeping my core warm without bulking up to the point that it interferes with my shooting.
What do others do beside run for camp and a hot rum when the mercury falls out the bottom of the thermometer.