I have an 11-87 that has a lot of peening right behind the locking lug groove on the barrel extension and the back of the receiver, I remember in the course it might be head space. I do not have a no-go gauge for 12 but I can use a good 12 gauge from federal and put a 20 thousand shim behind it and it will close on that. This gun came in for a clean job and it looks to have a lot of wear on it from duck hunting in the pond!! Is that accurate enough the way I checked the headspace on this gun?
That is the correct way to check headspace, and it sounds like you have some. But you say the peening is behind the locking surface, and the receiver. That does not sound like headspace. The peening behind the locking recess in the barrel extension is the gun unlocking and the locking block bouncing back up when the bolt is on its rearward travel. Check to see that the gun has all the correct gas system parts and that they are installed correctly, then check the receiver where the barrel slides in and stops. If the receiver extends down below the barrel the bolt can hit this when it is moving to the rear and cause the various vibrations and jumps and bumps that cause all kinds of problems.
