It is pretty easy for anyone to check their loads . And rifle. Put the ammo and rifle in your deep freezer for a couple hours, Set your chronograph up then grab the rifle and ammo and go chronograph it. That is provided you have your freezer real close to where you can shoot. Where I am I have to put the rifle and ammo outside but keep the chrony by the wood stove. , I get the rifle all set up then take the chrony outside, set it, turn it on and commence fire. At -40 I can get at least a 3 shot string before the chrony freezes up.
FWIW, this is the reason I switched to Hodgdon's extreme powders a few years ago. They are probably the most consistent powders when exposed to large temp swings like what you are talking about. I have been very pleased with H1000 and don't see myself changing powders again, very consistent.
I heard somewhere that once you get to these sorts of low temps that a hotter primer might help. I believe Winchester makes the hottest primers. Would be interesting to see if a Winchester WLR or even a WLRM primer helped.
Ya a hotter primer may help. And 40 below is cold
But it feels warm compared to 60 below which is real cold until it gets colder and starts blowing. As I type this it's 40 below at my house. And it really doesn't feel that cold. There are Lots of people who hunt up here in the cold.