Copper Fouling
Jacket metal smeared into the rifling that builds up over a string and degrades accuracy until it is cleaned out with a copper solvent.
Copper fouling is jacket material that shears off a jacketed bullet and plates itself into the bore as the bullet engraves and passes down the rifling. A thin, even layer can actually settle a barrel and help it shoot, but as the deposit thickens over a string it makes the surface rough and inconsistent, and accuracy falls off until the copper is removed. It tends to be most pronounced in rough or new barrels and in those that have not yet smoothed out with use.
Removing it usually calls for a dedicated copper solvent rather than a general-purpose CLP, since the chemistry that dissolves copper is different from what cuts carbon and oil. How far you strip it is a judgment call: some shooters clean to bare steel and accept a few fouling shots to reach a stable cold clean bore, while others leave a light layer in place. A borescope makes it easy to see exactly how much copper remains.