Here's my .02 cents worth,Use magnum primers for all ball powders and in any ammo used in -10F period! Your primer makes up as much as 5% of the loads velocity.Ever hunted in -40F? It's damned cold when your barrel turns white with hoare frost on your first shot and I like to hear it go bang.Every big game cartridge gets magnum primers for my Alberta hunting even the 30-30.Groups are the same and your lighting the hottest fire under your powder to ensure it burns in spite of the temperature.If your going to the tropics back off two grains and your good to go.Been handloading 25years and haven't any use for factory ammo save the brass.I feel I can exceed or at worst match any thing the factory sells cheaper.Thanx Harold