One other thing you might think about... As someone has already mentioned the 'ambient air temp' and the effects on the sabot.

I will just mention what I have found with no scientific facts.

For myself and in my case most of my hunting is in the cold temps that you refer to. The polymer formulation used in Harvester sabots gets very hard and looses some of its pliability. I have always felt that because of this the sabot does not really mold itself to the lands and grooves of the barrel. This same theory helps me explain why Harvesters appear to work better in the warm temperatures or in a warm barrel, when the Harvester is able to mold itself to the bore.

On the other hand I have found that the polymer formulation use be MMP works very well in really cold temperatures and because it maintains pliability it does mold itself to the bore.

One of the reason that I believe this other than experiences is loading a snug harvester or a snug MMP in sub zero weather. Often the Harvester will be shaved slightly as it goes through the crown and the MMP NORMALLY does not. And I would suggest the MMP is easier to load all the way down the bore than is the Harvester.

On the warm end of the spectrum the MMP gets very gummy in warm weather or in a hot barrel. Gummy is probably not the right word in that it does not stick to the bore - it just becomes very soft in the presence of abnormal heat.

Just sharing..