Nickel is a good, hard finish. Don't use anything with ammonia on it or near it. If your bore cleaner has ammonia in it, and it probably does, keep it away from the finish. If the finish is cracked, the ammonia will react with the copper below the nickel. Once I used the Flitz on it, and I wouldn't use that unless it is pretty bad, I would just use CLP to keep it clean after that.