If you look in the Nosler reloading manual, they show that with the 225gr. bullet, the "most accurate powder" they tested was Reloader 19. The most "accurate load" tested was with a max load of IMR 4831 at 72gr. shooting 2847fps. I have found in my rifle the most accurate load was with RL-19 shooting a half a grain under the max load of 72.5grs. I shoot 72grs. and get a little over 2800fps. out of my gun. Every gun is different, you will probably have to try different powders and loads to find out what is the optimum load for your particular gun. You have a longer barrel so a slower powder will work better for velocity, if that is what you are after. They tested these loads using a 24" barreled rifle. As far as RL-19 being temp. sensitive, it probably is but I havnt noticed it, I only use my 338 for elk in the high country when it is extremly cold. I have found the only powder that I have used that is extremly temp. sensitive is H335, I use it in my 223's. I have a winter load and a summer load because it fluctuates so much.