I believe you're right Muley, I have experienced "flash rust" over the years past when I used to flush with boiling soapy water and then rinse with straight boiling water. Nowadays when I've finished shooting I wipe the bore out with a few patches saturated with Butch's Black Powder Solvent (or similar) to get the heavy fouling out and then flush with warm tap water and Dawn dish soap. I then flush several changes of clean warm tap water through the bore and begin drying the bore immediately after I'm satisfied there's no soap residue left. I then plug the touch hole or nipple and pour a few tablespoons of 91% Iso Alcohol in the bore and with my finger over the muzzle, I tip the barrel back and forth a few times and then pour out the alcohol. I push a dry patch down into the powder well area and tamp it gently with the rod and then leave the barrel stand muzzle down for a few minutes. The alcohol picks up any remaining moisture from all the little nooks a patch can't get into. I fish that patch out with a worm and dry the bore with clean dry patches. Wipe the vent/nipple area clean with cotton swabs and run a wet patch of ballistol or barricade down and out. Wipe the outside off with my oil patch and done. I know it sounds like a lengthy process but it's not bad and I have never had an issue with "flash rust" since.

BPS