it is most likely oxidized copper fouling from being shot with copper jacketed ammo; CLP usually won't cut it; Hoppes 9 will cut it, but allow a few hours to overnight to work; Barnes CR-10 works fast, but is cautic to metal and needs to be neutralized and removed within @ 15 minutes; JB bore paste works good, but needs elbow grease and a few passes (does not embed the bore--not abrasive); Flitz metal polish works good, but go light since it is abrasive and it can etch the bore if worked in harshly enough; kleen bore lead away cloth works good, but if used too much it can remove the bluing from a carbon steel gun;
I reserve the lead away cloth for my stainless steel revolvers and use the JB bore paste for my blued steel revolvers; don't be suprised if it takes several passes to get rid of all the green stuff; one time I used Outer's nitro solvent cleaner (somewhat similar to Hoppes 9) and didn't get it all out...several months later while relubricating the bore of that particular rifle I had green stuff coming out on the patches since it sat so long that it got under some copper foulijng that got left behind after the previous cleaning
