Try scrubbing the bore--and also the interior of the gas cylinder, and the front of the gas piston--with dishwashing liquid, then rinse with boiling water.
What color is the fouling coming out? If it's black, that's carbon fouling from the propellant. If it's greenish or bluish, that's dissolved copper from bullet jackets.
Yes, some older milsurp ammo is that dirty, though it's also possible, since you got an old milsurp rifle, that it passed through many hands before it got to you, and no one ever tried to clean it.
After the boiling water rinse, try wetting the bore with solvent and then putting patches wet with the solvent in the chamber and in the muzzle. Put the rifle somewhere that it can stand muzzle-down without making a mess and let the solvent work overnight, or even for a day or two, and try again after that.
Aerosol brake cleaner or carburetor cleaner is very good at removing carbon fouling, though of course you will want to use that in a well ventilated area, plus the fumes are probably highly flammable, depending on brand and composition.
Current production noncorrosive 7.62x39mm brass case commercial ammo isn't very dirty, but if you shoot thousands of rounds without cleaning, of course stuff will build up. If you are writing from Canada and asking us about current Chinese 7.62x39mm ammo, about that stuff I do not know. I know the Chinese 7.62x39mm ammo we could get in the US before the 1993 ban was filthy stuff. The Russian commercial steelcase isn't much cleaner.