Definitely sounds to me the extreme cold weather affected the pressure curve. When it is very cold, the powder's burning rate is affected slightly, hence preventing the gases from expanding like they would in warmer weather. Basically, the cold weather causes pressures to run lower, and that contributes to carbon being deposited inside the chamber/on the cases. The lower pressures will not force the brass into the chamber's walls completely and seal off the gas's ability to enter the chamber. Reverse could occur if a good load was developed in colder weather, and would be taken and shot in warmer weather. Watch for pressure signs, as the powder will now burn more "readily" and pressures could spike above the standard limits. I read that when loads are going to be fired knowingly in extreme cold conditions, it would be better to work up loads using magnum primers to ensure a more consistent ignition which would contribute to more reliable pressures and accuracy.
-Patriot
