There have been more than one post on this forum about the corrosive properties of BH209. I don't really recall the particulars, but it seems to me it has been mentioned that sulfur is part of the make-up of BH. I believe that sulfur mixed with water wants to cause rust in a rifle barrel. Myself, i know nothing about the chemical properties of BH, but i do know that if left uncleaned after shooting BH, a rifle barrel will rust. More than one of my rifle barrels have minor pitting due to leaving them uncleaned after shooting BH209. Removing the fluff with a dry patch will help. Perhaps reloading, and pushing the fluff into the powder will help stop this rusting.
What i do is load my rifle at the beginning of season, and leave it loaded until i get a shot. Then i reload, and if this second shot isn't needed, i leave the rifle loaded until the next time i need a shot, or until i know i won't hunt the next day. The vast majority of the time, the barrel is fine, and has no rust. However, ever once in a while, there ends up being some rust in the barrel. This results in minor pitting. Accuracy doesn't seem to suffer, but the barrel has minor pitting. Myself, i don't understand why the majority of the time, the uncleaned barrel doesn't rust, but on rare occasions it does rust. As i type this, my rifle is clean, and in the truck loaded.