Be sure to watch what you use for bore cleaning, do not use anything that is formulated to remove copper fouling, which usually contains ammonia.
Under that nickle plate is probably a layer of copper... that is usually done because nickel plates to copper better than high carbon steel.
The bore cleaners containing ammonia will eventually attack the copper, and cause the nickel to lift/peel. If the nickel surface is comprimised in the least, the cleaner will undermine it.
It can also cause the nickel to discolor/stain.
Breakfree CLP sounds good... what ever you plan to use on the gun, just be sure to read the label for any warnings about using it on nickel.