Yeah MM, I've experienced that with my Omega X7. It really bothered me the first few times I shot it. It's a little difficult to get at and I would spend a lot of time with Q-Tips and solvent trying to get it squeaky clean. Then I took a good look at it with a magnifying glass and came to the conclusion that it's mostly staining and/or copper transfer from the primer housing. Now I just give it a good wiping with a Q-Tip and rub a little oil on the breech face. The ring you're talking about remains clearly visable, but there's no crud build up after the seven range sessions I've had with it.