One of the old Gun Digests had quite an article on cupro-nickel fouling with the first 06 ammo. It want till the later gilding metal was developed that the fouling problem went away. If that is what is there it is about 100 years old. Krag ammo was loaded using the same bullet jacket alloy but the lower velocity caused no problems.

If that is the old fouling still there it will be exceptionally hard to remove.