It probably isn't a factor in your country, but in one of my tests H100V lost more velocity in cold weather than any other powder I've tested, 186 fps from 70 to zero. Now, this was with 165-grain bullets in a .30-06, and temp-sensitivity is to a certain extent dependent on the specific application, so it might not lose as much in your .257, and as I noted it may not be a factor in your hunting.
