It is as you described Ike. Conventional rough and/or finish chamber reamers are used. I suppose that, somehow, a barrel made it past initial QC that was improperly formed... I just don't see how it could go through several following steps, unseen.
I've heard of cold hammer forging mandrels that include the chamber... talk about your concentricity! But I believe those are very, very rare. Technologies change quickly, so I could be wrong about that these days... but I don't think so.
C
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