I use a bore pig setup to control fouling after each shot. But I don't use a bore mop to follow the bore pigs as they tend to gather moisture and only get the chamber dry for a few shots before they're saturated. So I use a dry patch in the chamber, and down the bore after each pass with my bore pigs. My bore pigs also aren't soaking wet, just damp. I dip the felts in my mix of NAPA water soluable cutting fluid and water and then put them into plastic cartridge boxes before heading to the range.

I was just a child in the early 60's. This was a fair bit more 'spensive...

There's a Persian 98/2x 8mm that's as nice or better. Ran into the seller at a show, a buddy of his would sort thru them and ship him the nicest one.

That Persian 8mm ... 'yeah, that one was nice. You should have seen the first one he sent, had a stock like a Weatherby... UPS broke it in half, I nearly cried'.

I am ignorant to all things pertaining to black powder and lead bullets but how can a damp chamber cause this to happen.
I suppose it must do as it doesn't happen if the chamber is dry.
A few years ago now I bought a custom made highwall in 7mm/303 imp. The guy I bought it from had also bought it from somebody else and had only put a few rounds through it and used the loads given years ago from a well known writer who I can't now remember his name due no doubt to loosing my marbles with age.
As I never use somebody's else's reloads which he gave me I cleaned the cases up with fine wire wool before pulling the bullets and this is what happened.
I thought perhaps he hadn't washed the cases after sizing and left lube in the cases which caused the corrosion on the brass.
The rifle came with custom made dies which fit the chamber perfectly and thanks again to Bulseytom who with his computer worked out some loads using ADI powder as I couldn't find anything on the internet.