Luke,
John Sabato actually posted this a couple of weeks ago in a discussion about making Luger barrels.
To see the ring, simply dismount the cannon and look into the breech end in the conditions under which you would normally inspect rifling. In my experience, examining my own Lugers and a couple others I considered buying recently, the ring is sometimes very prominent, and sometimes can be difficult to detect. Also, I have seen it not quite completely circumferential. I don't know what this might be attributable to, unsubstantiated speculation might indicate wear or manufacturing variation.
--Dwight
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