I've been cleaning ML barrels since 1972. The only barrels I have EVER seen flash rusting in, is the TC barrel I started with, and one of Les Bauska's barrels - while I was still using the HOT water method of cleaning.
I repeat for the umpteenth time - Holland and Holland, the longtime Gun Manufacturer for over1 1/2 centuries (not the brand new Bl. maker in the States) stated in a letter to a friend of mine how he should clean his Holland&Holland, Rigby, John Blissett, Wesltey Richards, Ward, Army Navy and some other makes of double barreled black powder rifles and shotguns. Will, RIP, was a double gun collector who sent EVERY one of them to H&H for refubishing- and, he asked H&H how to clean them properly. H&H responded as thus.
Their instructions were to use water from the cold water tap. Real black powder fouling is dissolved by cool or cold water. Boiling or very hot water on the other hand actually has a glazing effect on built-up fouling - overheats the barrels and also causes flash rusting which is accumulative and eventually turns into the fine pitting.
You can see that in many black powder guns- one end to the other.
Do you suppose THOSE were cleaned for decades with boiling or VERY hot water? Quite possibly, or probably. Once screwed up by repeated flash rusting - your bores will always "foul" and need wiping while shooting - oh -" it's just so hard to load with your combinations"- wonder why? Do you use hot water for cleaning or not remove the barrel for cleaning? My suggestion is to re-barrel it and do it right from now on.
Since I started using this cleaning method - I've NEVER had any rusting in my ML barrels - flash rusting or other - none.
Now, Holland and Holland should know what they are talking about. They are the company who totally built the guns in the 1800's, locks stocks and barrels, which today sell for many tens of thousands of $$ on the used market and who make modern double guns today, rifles and shotguns that retail for over $100,000.00. A ten bore side by side hammer gun form 1884, recently sold at auction for $80,000.00. Maybe they know what they are talking about?
Cleaning a ML barrel does not take very long. In fact, it actually takes less time than is needed to properly clean, that is cleaning all of the smokeless powder fouling and copper fouling from a modern commercial rifle's barrel.
Different areas having different humidity which need different preservative measures applied AFTER cleaning and drying, to keep rust at bay when firearms are stored.