Copper fouling

[Mr. 16 gauge]

I usually do not have an issue removing copper fouling from my rifles, but my surplus M96 mauser is giving me fits......I have NEVER been able to get rid of the copper fouling in this bugger since the day I bought it. I don't know what the problem is....I scrub and use shooters choice bore cleaner and the patches continue to come out just bluer than blue! After my antelope hunt, I decided to clean it until it came out clean....that was Three weeks ago!...I have been running a wet patch through, waiting an hour or two, then running a dry patch through, then repeating the process, and I am making absolutely NO headway whatsoever.
Any thoughts? I'm wondering if the fact that it is an older military rifle and most likely was used with corrosive ammo in the past might have something to do with it. More importantly: does anyone have any suggestions? Someone at work mentioned a 'foam' that you squirt in the bore and leave overnight, and supposedly cleans the bore "immaculatlely".....he didn't remember the name, and I can't seem to find a product that resembles it.

Any help would be greatly appreciated.