Re: first 7mag loads
Yes, the alliant powders such as R25 & R22 are more sensitive to altitude and temp. They usually give higher velocities, but the inconsistencies that I have experienced with them have made me shy away from them and start using Hodgdon Extreme powders. I shoot the big 30s such as the .308 Warbird and 300 RUM, and with Retumbo, I'm getting consistent velocities and performance.
I'm not sure about the IMR powders, but if that is what Remington is using in their factory ammo, I will never use it. I was led to believe that Rem maybe using 7828 for the 300RUM, and I had extreme spreads of +100fps through ONE BOX of factory sciroccos.
Lastly, the most important thing to do between your load testing, is CLEAN YOUR BARREL...you maybe already aware of this, but it should be top priority when you are shooting bullets that are constructed of different metal alloys. I had a hell of a time trying to get Barnes bullets to shoot after testing ballistic tips in my Warbird. I cleaned my barrel and voila, got my first 1/2" group with that gun and barnes XLCs.
Good luck with it all, when you do find the load that works for you, you'll be on cloud nine.
God Bless and Shoot Straight