Kroil is the secret to removing copper, or any barrel fouling for that matter. It is a very fine oil - the oil that kreeps!  Some have even complained it kreeps so well the metal can won't contain it after a while.  I have a good Kroil stain on my shelf from this. The can has a "for industrial use only " comment on it but it has been available through automotive stores and larger outlets selling gun cleaning supplies for some time now.

Serious benchrest shooters were the first to use it to as barrel cleaner to remove fouling. Swab some down the bore and let it sit overnight and it kreeps under the fouling  which just brushes out.  Nasty fouling may take a couple applications.