Many of the powders are very water hungry.Some of the worse powders are the American Pioneer Products they claim. Although I have never left an APP load in so I will not claim it as gospel. I personally think all powders will absorb moisture from the air if they are given the opportunity. How much and what will happen is the question.

I hunted with Pyrodex RS one year and left the rifle loaded that night, but stored in a garage (so the temperatures would remain the same). The next day I had a shot at a nice little buck. When all that fired was the cap, I could have cried as it was the only horn I had seen in 12 days of hunting. That is when I decided no more leaving them loaded over night.

There are some people that have real good luck leaving them loaded. I am not one of them. But you can decide what you want to do. I shoot them off at the end of the day, or pull the charge, and basically clean the rifle and start new the next day. Others put the rifles in the garage, car, shed, even in the house, and never have a problem. Lucky is not my name.