Removing some type of bluing from Damascus

I was wondering if anybody could tell me the best way to remove bluing from an old damascus shotgun barrel. I know bluing is sometimes used to enhance the twist pattern in the barrel, but in this case my grandpa decided to have someone blue the barrel and you can no longer see the twist pattern. It looks like it was dipped in some kind of thick brown stain. I don't want to ruin the gun and was wondering what lightest to most drastic way would be to start removing the substance.

Hope that makes sense thanks for the help