Builder, all my choke tube guns (both of them) get some gas stain and I've been shooting them for more than 20 years in competition and casual shooting. It is impossible to seal at the back end without hand fitting and some gas stain is part of the deal and harmless. My Beretta and "Briley for Beretta" tubes have been shot tens of thousands of rounds trouble free and are usually a little dirty when removed. The Lovely Linda's Krieghoff has Briley installed screw ins and it's the same situation.