TheLeadPot is ‘dead nuts on.’
‘bout 6-9 months ago there was a similar thread over on the Shiloh Sharps board. Kirk, the owner of Shiloh Sharps, reported that utilizing a metal cutting band saw he sectioned a badly pitted barrel. From what he observed, copper fouling formed over the BP fouling & the BP residue did its nefarious work….
If you do shoot copper & then change to lead, you need to very scrupulously clean the barrel. Any copper fouling will attract lead. There is a reason why the cast bullet bench rest shooters don’t shoot copper in their rifles.
Some opine that shooting copper will remove lead fouling. Numerous years ago, the NRA published an article on this method. Using a bore scope & examining patches, the NRA staff found that the copper jacket just smeared the lead over a greater surface area &/or the leading caused copper fouling to build up over it.