If that is the "electroless" nickel, I had a devil of a time trying to clean it until I stripped the gun and put the external parts in the dishwasher. Cascade worked.
As to Hoppe's, it is an ammonia solution that can attack both nickel and the copper used as an undercladding on most nickel plated guns. It won't normally discolor nickel unless left on, but if it can get to the copper the nickel will peel.
You might try gasoline (please be careful with flames!!) and see if it will remove the discoloration.