Since I can't sleep tonight, I thought I would spend my insomnia moments offering my assistence.
Alliant's has data for 300 MP uses a Federal #100, which is a standard small pistol primer. But I feel confident that most would agree, Federal primers would not be the primer of choice for high operating pressure loads. This is primarily because they are so soft, thus making them suseptable to leakage or punching when exposed to full magnum operating pressures. My initial choice here, would be a CCI 550, with my second choice a Wichester small pistol magnum.
The best possible powders capable of achieving your performance goal, are going to be slow burning types. Full house loads, and especially those being used with heavy jacketed bullets, are going require slow burning powders, and magnum primers for cosistent and safe operating pressures.
2400 is certainly capable of lower to mid range magnum performance, but to try and achieve maximum obtainable velocities is going to put you on thin ice regarding operating pressures when topping out at maximum obtainable performance. It's a powder many love and use for achieving magnum performance, but it still falls a little short of what 296 / H110 can do at mid range data, and without unpredictably pushing the pressure envelope. So stear clear of fast burning powders, and even mid range burn rates aren't even going to get you there without some elevated pressures concerns.
I lived in South Dakota for about 7 years, so my experience with sub zero operating conditions and slow burning powders is on target to your concerns. In this regard, any powder that fits the bill, is going to require a magnum primer to provide acceptable ignition, and consistent burn characteristics. The extreme cold enviroments you contend with, are clearly justifiable concern. And for those who have never lived and shot firearms in conditions as extremely cold as South Dakota, my first experience with minus 80-90 degrees below zero chill factors, froze the entire firing pin mechanism in under 1 minute on my Rem. 870's. My Son's and I tried everything we could think of to eliminate the issue, but to no avail what so ever. Nature won that battle hands down!
Now for consistent full house magnum performance, 296/H110 with a CCI-550 primer has met my full hous magnum goals without ever experiencing a single issue for more than 30 years now. 296/H110 are specifically designed to get you maximum obtainable velocity while safely doing so. In fact, the work up I perform with those powders, is unrelated to pressure. I have justifiably always felt safe just pickng the charge I want for those two magnum propellants because they don't really produce much of a noticable difference within the published load range. Do not mis construe that statement to imply that skipping a proper work is not necessary with them. I'm only relating my personal experience with them.
GS
You've for something backwards there.
