Before the nickle plating is applied, it's copper plated first. IF there is any kind of ding in the nickle, Hoppes will get under it, and dissolve the copper between the steel of the gun, and the nickle. Main reason you see a lot of older guns with the nickle flaking off.Hoppes # 9 specifically says, "Do not soak nickel-plated firearms in No. 9. Wipe nickel surfaces dry after cleaning".