If you have been shooting ammunition with jacket bullets, it is probably streaks of bullet jacket metal. An aggressive scrubbing with a special bore solvent designed to dissolve copper build-up should return it to its former silver-colored stainless-steel appearance.

This is one of the few times I might recommend buying and using a stainless-steel bore brush, but I would only use it to clean the chambers in the cylinder, NOT the barrel. Not only will the SS brush make short work of cleaning out the fouling and last for a long time, but if you were to use a standard copper/bronze bore brush, the brush itself would also be attacked by the special cleaning solvent, so you would probably only get a single use out of it.