I learned the hard way about using Magnum primers in cold weather. I had loaded up some 7MM-08 140 Grain Sierra Game kings/with H414 with standard primers and then after sighting in took the combination to Maine for my annual deer hunt. Well the temperature got down to about 10 below one morning and as I was walking over a small ridge I spotted an 8 pointer about 15 yards away, bedded down no less. I raised my rifle, the deer got up and I fired. Instead of a bang I got a weak report and the deer left rather quickly. Upon checking I found a lot of unburned powder in the barrel. From then on I used magnum primers for that particular combination. Never had another problem.I use mag primers with ball powders and some of the reloader powders in colder weather to offset the temp instability.