Lyman GPR in .54
I can’t tell what this marking is in my barrel. I’ve never dealt with leading before, but a piece of Birchwood Casey lead remover cloth on a brush, as well as 0000 steel wool won’t take it off. From what I can tell with a flashlight, it appears to only be in the first 2-3 inches. I keep it oiled up good after cleaning, so it seems bizarre that it would be any kind of pitting. I can’t tell if there’s any color to it, the best way I can describe it is that it looks like frosting (like on a bearing).