Living in Wisconsin I might have some of the same weather as you experience in Minnesota. Leaving the rifle loaded length of time, depends on a lot of factors. First the kind of rifle you are shooting. Next, kind of powder I am shooting. Also, how easy it is to unload the rifle.
If I am hunting in wet weather, be it rain or snow, I unload the rifle, clean the rifle and start new the next day. I am not taking the chance that the rifle will fail to fire come crunch time, simply because I was unwilling to put in ten minutes worth of cleaning time the night before.
If I am hunting with an inline rifle.. then unloading is so easy, I figure why not. Remove the breech plug, push the load out with a wood dowel, and clean the barrel. Then reassemble the rifle.
If I am hunting with a traditional rifle, they can be a little more tricky. The easy way to unload is of course fire the rifle off at the end of the day. Otherwise you have to pull the charge. Depending on the kind of projectile, it can be rather tuff to do. For instance, a sabot is much harder to pull then a roundball. The CO2 dis-charger is a blessing here. Also I have tried to unload them with an air compressor. Sometimes it works sometimes it does not. It is just easier to shoot the load out and clean the rifle.
If you are going to leave the rifle loaded, be very careful of the temperatures you subject the rifle to. Bringing a rifle in from bitter cold to a warm area is almost a sure guarantee that you might have an ignition problem the next day. Also the place you store the rifle, pick an area that is not real humid. Even though the temperature is the same, there is no use in subjecting the rifle to humidity.
First of course, is remove the primer, #11 cap, or clean the pan. No use in taking any chances. Take the muzzle mit off and wipe the barrel outside down with an oil patch. Then swab down to the projectile with dry patches. We want everything dry in there. No use leaving anything that could be damp, in there. I then like to put a cotton patch in the breech over the breech plug. The thought here is to absorb any moisture that might form in that area before it gets down the breech plug or into the nipple area of the rifle.
I then set the muzzle on a clean dry cotton cloth or newspaper, leaning the rifle in a corner so the charge is higher then the muzzle. I tie a red bandanna through the trigger guard to warn anyone that the rifle is loaded and to leave it alone.
The next day, I swab the barrel again with a dry patch and then with the ramrod, check the load to make sure it has not moved off the charge. I then put a NEW cap or primer on rifle.
As long as you do not shoot the rifle, which starts the powder rusting process, you can leave it loaded as long as you like.
