The rifle is less critical than the lubes and amounts of lubes you use. The metal itself won't care to any temperature you could survive.
I have used sidelock muzzleloaders to -15°F (in NY primitive m/l always came after regular season in late December). WAAAAY too cold to sit out for very long. But I did. Gutting the buck was like sticking my cold, stiff hands in boiling water.
Look up "The Winter War" of 1939/1940 in Finland. Rifles work very well in the cold if you lube them lightly.
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