Up here -40 F ambient, we don't count windchill, is pretty common. It was -48 F a week ago, only -30F this morning. It doesn't hurt powder at all. We do however always use magnum primers in all hunting loads cause going bang is quite a bit less certain colder than -30. Interestingly, speeds do not slow down near as much as one would think at -40. I have loads that shoot within 50 fps of what they do at 70 F. Very good way to check how temp stable your powder is, and how good your primer choice works. A lot of loads that show large FPS drops are the result of the primer not being hot enough.