First off, it is a nickel barrel. A blued barrel with a fancy finish. I have a nickel Staghorn Magnum. My rifle developed a stain in the open breech area. I have tried a number of things to remove it, and while I have reduced it, it is still there.

Blackhorn 209 might have caused that, but really that false muzzle should be given special attention at cleaning time. All too often it is not scrubbed well because of the over size. I would try some sort of a cleaner like you mentioned. I never used the one you named but I have used others. Hope it comes clean for you.