My favorite rifle is a lefty 40 cal flinter built by Jack Patterson from Upstate New York. Beautiful rifle, and a great shooter for some years. I don't know the history of the rifle, only that I am the third owner. I know I have put many pounds of powder and lead through it.
Recently it seemed as though accuracy had dropped off, and I was getting an incredible amount of fouling sludge buildup. Got lots of advice from the long term shooters at Ft Lupton ML Club. Learned something- ML barrels wear out in two areas, the muzzle and the area where the load sits. Having been religious about use of a rod guide, the crown looks just fine, and fired patches are pretty.
After really cleaning the barrel well, with several scrubbings and flushings I ran a tightish patch down and sensed a loosening in the 2-3 inches of the bore just ahead of the breech. What to do? Experimented with 1, 2, and 3 lubed hard felt wads between powder and ball. This moves the seated ball forward into an area where the rifling is still good. Accuracy is mush better. Now I have to work more on the big variable- me.