You need to try Sharp Shooter Products "Wipe Out" and/or "Patch Out" - with or without their "Accelerator" added. I had a black bore M1917, too. I tried regular Hoppe's , old fashioned cleaning - just got black mud. I used Wipe Out - let it set overnight - got a ton of Black goop. Then foamed the bore, got a bunch of copper blue on patch. Then foamed it again - got black goo again - then foamed again - copper...
There were FIVE distinct Layers of fouling black over copper - that responded to Wipe Out - and NOT to regular cleaners. I think Sportsman's Warehouse stocks it. Check Cabelas and Brownells, or order direct from the maker in Kansas.
CC
PS - Brownell's sells a famous paste bore cleaner that is noted for this kind of problem - can't remember the name - look under bore cleaners. CC
Last edited by Col. Colt; 12-20-2018 at 09:59.
Colt, Glock and Remington factory trained LE Armorer
LE Trained Firearms Instructor