Well I just had a similar experience with bad copper fouling in a rifle I purchased used at a gun show recently. My findings:
(1) Regular Outer's does almost nothing.
(2) Outer's Copper Remover Works minimally.
(3) Butch's Bore Shine and
(4) Shooter's Choice Copper Remover worked fair
(5) Sweet's 7.62 Solvent got serious about the job. Follow the directions!! I then finished up the bore with J-B Bore Cleaning Compound. Once again, follow the directions, and be patient. It takes some time and elbow grease, particularly with the solvent remover.
Good scrubbin', and good luck................