Running the #'s , my .54 rifle [ Colerain 42 in " C" weight swamped barrel ] is approaching 15,000 rounds. My target load was 80 gr of FFg . Somewhere around 8 to 10,000 groups opened up, . Upping the charge to 90 gr brought it back , but it's now shooting larger groups again with a lower point of impact. The rifle is now used for hunting, with a 100 gr load.
I've heard that roughing the bore can help ..the idea being that the bore becomes too slick, but I've never had the nerve to try it. A proper range rod was always used , and clean up is with room temp water, followed by WD 40, and bear oil
Lube was WD40 [ liquid, not the spray ] or bear oil. I should say that this rifle has been with me for 30 years. I used to shoot a .535 ball, but in later years switched to a .540 from a Green Mountain mould.