Wipe-out is easily the best copper-specific solvent I've used.

I have one of the late Winchester M70LH Safari Express .375H&H's, and from new, it had awful copper fouling - solid streaks along the top of the lands. Sweet's took ages to get it out, but Wipe-out would remove it all in one or at most two applications. After nearly 100 rounds [shoot 1 or 2, clean, repeat], that barrel is finally getting run in, to the point of only fouling at the land edges.