Unfortunately, the paper likely attracted moisture, and the metal under the paper rusted.
Would start by wetting the chamber with a good penetrating oil. I am partial to Kroil, but there are others. Let soak overnight.
Take a .50 cal borebrush, wet with some additional penetrant, battery powered drill, slow speed, moving in and out for about 60 seconds. Then mop the chamber, inspect, re-wet, soak and repeat as needed.
If the chamber is rough from rust (pitting) you MAY be able to make it better (not good, just better) by using the drill and a VERY mild abrasive such as JB Bore Shine, or even toothpaste. Be sure to remove any trace of the polishing product after use. Lightly oil chamber.