Me, the only powder i shoot is Blackhorn. No way would i leave my rifle loaded for an extended time after a shot. My first shot whilst hunting will be taken from a clean shiny barrel. The first thing i do after taking a shot at a critter is reload. If i am hunting from home, i fire the load when i leave the hunting area. If i am camping, i leave the rifle loaded for the duration of the trip, maybe, a few days, then discharge the load when i leave for home.
Three of my rifles, or ex rifles, are testament to the fact that burnt Blackhorn residue is caustic, and i do what i can to insure there will not be a fourth..
For those that are concerned about 'that first shot', let me suggest you try shooting your rifle once, then swab with a dry patch once, then load it for hunting. This will remove the loose residue, and thus maybe prevent moisture being absorbed by the residue, combining with the chemicals there, and causing the rust pitting i have experienced. Having never tried such a thing, i really have no idea, if it will help with the issue of 'that first shot'.