I have noticed in my premium barrels the the absence or presence of copper do not appear to be copper that is stripped by the barrel, but copper that is deposited by the gas that is produced, and settles in the bore. Different Bullets/powder combinations seem to produce different levels of copper deposits. If you drop your muzzle brake into a small vessel of copper solvent(BoreTech), it will turn blue in a matter of minutes......solid proof of copper/gas fall-out. The Hodgdon powders combined with Berger/JLK which use the same jacket material) generally seem to produce the least amount of copper residue .in my premium barreled rifles. Pressure of the load also seems to influence this as well. IMO.