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.
What we have here is... failure- to communicate.