Jim, Welcome to our site. What you describe is commonly called "riviling". It may aor may not be a through and through annular ring. Usually it is confined to the inner surface of the barrels. Usually causes no problems, can be honed out but does not seem to be worth the effort. Several theories as to etiology from partial obsturction to "weak"/thin barrel wall. I've seen it in Parkers, Winnie 101's, and LeFever barrels. Some place I have a nice long technical description which I will post if I can find it. Best, Dr. BILL