IMO, H4350 is where to start with the 7x57, and especially with the 140-150gr bullets. When it comes to temperature sensitivity and IMR 4350 (and actually most powders), I usually ignore that notion. As long as you develop a load that's safe in the summer, it will be fine in the winter. And, it will hunt just fine too. In my personal 30/06, I use IMR 4350 with 165s. I try to avoid shooting in the middle of the summer, but even early fall where I shoot it's often over 90 degrees in the morning. Last buck I shot with that load was on a 16 degree morning, and at 200 yards that bullet landed exactly where I intended. Your mileage may vary, but I doubt it. Getting back to the 7x57, if 4350 doesn't work in your rifle, W760 with something in the 150-160gr class of bullets works well. If you are a max load type of shooter, I FOR SURE would develop a load in the heat of the summer with that powder. It will be safe in the winter, but not always the other way around.
One more note about the 7x57 - don't ignore the "in-between" bullets. People tend to shoot 140gr or 160gr in 7mm, and sometimes 150gr. I had a finicky 7x57 that shot all those bullets decently, but dropping a 145gr Speer produced clover leafs. Same with the 154gr Hornday - not quite clover leafs, but more consistent that the 150 below and 160 above. Good luck!
