With all the moisture and higher humidity levels this summer I have been running an oil patch thru the barrels of the black powder shooters every couple months, used or not. I found one small area in a barrel that had some surface rust starting in July and that prompted me to keep on this. Just finished doing all of them again this afternoon and all is well. This is the first time ever that I have seen any rust in a barrel and have to think the weather has just been a real bear for high humidity and perhaps that’s what got it started.
Maybe check your barrels to see if you are getting any rust starting.