Ever solder any copper pipe?
Lead and copper stick together really well.

ALL the copper should be cleaned out of a barrel before shooting lead bullets, and ALL the lead should be cleaned out before shooting jacketed bullets. A bore that looks clean but a little dull still has copper fouling in it.

Using a jacketed bullet to "clean out" the lead raises the pressure (possibly to dangerous levels) and leaves a thin mess of lead and copper in the barrel that is harder to clean out than either alone.

A proper lead load shot from a barrel with no copper in it will leave little lead residue, if any. A model 14 I've used with 148gr lead wadcutters for bullseye shooting since 1970 still looks new.