I was out with my Parker SxS this year in -30C weather. Gun fired OK, #4 buck still kills coyotes. The gun was torn down in the fall, all the internals given a wipe of 0W20 Mobil 1 synthetic, and reassembled. Then the outside gets waxed with whatever hard wax I can source from the local auto parts store.

Break-open single shots do about as well, except I'm too lazy to tear them down: I just flush them with mineral spirits and lube with a drop or two of Kroil. Guns with external hammers usually have enough impact to fire, if they are clean and not gooped up with grease etc.

Savage 219 rifle (30-30) and similar rifles seem to need some detailing in the firing pin channel: anything in there seems to slow or jam the firing pin.

Don't know how much of that will transfer over to some of the guns with closer tolerances, but I would expect them to be more problematic. In seriously cold weather, I want a milsurp Mauser. I'm still trying to figure out how to keep my 1100 functioning in sub-freeing weather.