I think what I'm going to do is try turning off the gas flow via the adjustable block, and double check that these cases aren't being marred by pressure.
I am thinking if you are running different kinds of ammo through it with the same result it should not be a pressure problem.
I am opposed to the O ring and don't believe they are needed either. The insert and spring the way it was used for decades in the military has always worked well for me.
If that is not the issue I would check the tolerance between the extractor and the bolt face.