RE: jacketed bullets and Black Powder.
A few years ago, Kirk, B. owner of Shiloh Sharps, authored a thread on the Shiloh Sharps forum about this. Bottom Line: a shooter ruined a Shiloh Sharps barrel from doing this.
During the re-barreling process, the barrel was band sawed in half length wise. It was deeply pitted. From what Kirk said he found that the copper fouling covered the BP, which allowed the BP fouling to do it nefarious work on the steel. Seems that the copper fouling easily built up on the BP fouling and vice-versa. From what Kirk posted, a Shooter would need to take time to clean the bore of both the copper and the BP fouling and then periodically check it to make sure it all had been removed.