Hh, the chamber extension ring is in the receiver , the end of the barrel butts up to it when you place the barrel assembly in the receiver and turn it to lock it up. So there is a circumferential seam about a 1/4" in frt of the bolt face, this not a problem unless someone takes the gun down and starts polishing the barrel chamber with abrasives too much. It only takes a.001 or .002" larger in the barrel chamber than the ext ring to cause the problem I was referring to. When the high brass shells are fired the brass expands out larger diameter than the chamber extension ring in the receiver this what makes for hard extraction or plain having the hulls hang up. Someone who has owned a m12 for a long time and did his cleaning from the muzzle very rarely would have this problem. But cleaning from the chamber end is a bad move. I could not feel the seam with my finger nail but the problem was still there on the last 2 m12's I bought damned if I buy another. MB
* I’m not saying it didn’t happen/or you aren’t having this issue.
However, that sounds like some jack leg, card shooter, over bore mishap. Most brass has FAR more variance than that, and most any repeater will overcome a couple of thousandths brass malformity. It’s be hard to fathom removing the steel ONLY in part of the chamber, to a point of function issue, without power tools and stones or cutters, but that’s just what I’ve seen. While I don’t consider myself an M12 ‘expert’, in 40 years of owning, tinkering with, and being around M12s, I’ve never heard of that.
