This past season has taught me a few things about keeping my gun working when hunting on cold and snowy days.
Beating the cold:
If you do not unload your muzzleloader every day at the end of your hunt never allow your gun to warm up once it gets cold. The reason is you will get condensation in your barrel meaning you will have black powder + water = a gun that will not go off when you have your sights lined up on dinner.
Beating the rain:
Rain is probably one of the toughest things to deal with when it comes to flintlocks. A simple solution that is non-traditional, but works is to put a plastic sandwich bag over your lock. The traditional method would be to use a cows-knee which is pretty much a leather bag that goes around the lock. While I have never used a cows-knee I have heard that they work quite well. Another method that some people use is to put some sort of grease around the outside of the pan in such a way that the water cannot flow into the pan. Sometimes the grease method might be used in conjunction with one of the other methods which I would imagine would work quite nicely.
Unfortunately when it is raining there is also moisture in the air which well wet your priming powder. The solution for this is to simply check your priming powder every so often and change it when ever you think it needs changed. The tough part about this is that it can be a challenge to put new priming powder in without getting it wet while it is pouring rain out. Another thing that you can do that will help your priming powder last longer is to use a larger grain size. Sure 4F lights fast, but it also has more surface area to collect moisture so when there is humidity in the air you might want to use some 3F instead. Even 2F powder can be used in pan–that is what I use when hunting simply because it is what I have in my powder horn.
When ever it has been rainy out, or for whatever reason you suspect you gun might not fire, it is always a good idea to empty your gun whether you shoot, pull, or blow the load out, because you do not want to miss a shot because your powder was not dry.
Beating The Snow:
Snow is a little different than rain, because if it is a light fluffy snow it is not wet at all so you have no moisture problem in your pain right? Not unless you are removing your load after every hunt and bringing the gun inside. The trouble is when you take that warm gun out into the cold the snow hitting the gun will melt into water. So to keep your pan dry you have to tuck your lock under your arm or keep it covered with your hands (a cows-knee or plastic bag would work much better here). Now your body heat will keep your lock warm and the snow will keep melting and possibly you will get water in the pan. The better way of doing this is to put your gun outside out of the snow about an hour or so before you hunt begins. Now the snow will not melt when it lands on your gun and it will fall off when you turn the gun upside down. This works provided the snow is not a wet snow if it is a wet snow you will still have to cover the pan.