If I recall correctly, the primer in the Big Game load with 286-grain bullets is the CCI 200, which apparently works very well, even at zero Fahrenheit. I would suspect the reason is the charge is pretty compressed. (Well, that and the fact that my CZ 550 shoots just about anything accurately.)
But if a Big Game load doesn't shoot well with standard primers, I try a magnum primer. So far it's shrunk groups every time.
