Actually ANY residue of firing is hydroscopic. Corrosiveness comes with the territory alright, and true pyrodex is probably one of the worst, and I have a few comments here.
I also foul my barrel before each hunt, but I do it the morning of the hunt. I foul it if I can with a bullet on the powder, but if not, then a full charge packed and covered with a wad. Using a squib load for this with a sidelock can spell disaster if you are not careful, and I don't want to deal with fouling plugging up my nipples or fire channels like I have had happen in the past. A heavy charge takes care of all oils that may remain after an alcohol cleaning and numerous patches downbore. I also use pipe cleaners to make certain of a clean channel and a nipple prick to make sure that it too is ready.
Unlike you all though, I NEVER leave my gun overnight loaded by choice, cleaning and preparing for the following morning is A OK with me (Although my last bear hunt was an exception beyond MY CONTROL on the subject).
If you foul the bore, you can minimize the corrosion/hydroscopic actions by just wiping the bore ahead of the bullet with alcohol patches followed by dry patches and even a lightly oiled patch overnight to be wiped clean in the morning.... (WD40 might not be the choice for this) but even leaving it dry won't hurt.
Aloha....
