First off, I doubt if they ever let them get quite as fouled as we do today. Unless in combat whcih was really rare, I think they may have shot at a game animal and then cleaned in the field. One reason I think that is that the best way to get a misfire, especially with a percussion is to shoot it and then load without cleaning and let it set for a day or two. They left the guns loaded for some time and powder and ball was pretty dear such that shooting off a load at days end would not have been considered frugal. Greased patched ball may have been as much a barrel preservative as an aid to accuracy as it greases the barrel when seated. Tow was commonly used, which to the best of my knowledge is the fuzzy stuff off of linen or maybe wool, kind of a byproduct, that is not suitable for the loom. They had a jag for tow which appeared to be much like a patch puller where they could wrap it and for a ball to clean with. Barrels were iron carbon and did not contain the alloys of today. However I believe they were very low carbon, which is why they may have been so thick. Low carbon does not rust as badly as a higher carbon, wrought iron with no carbon barely rusts at all. Freshing a barrel was a very common gunsmith chore, whcih implies that they may have had some cleaning issues.
DP