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CM is even harder then SS
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I'm not sure that this is true all the time. CM and stainless grades used in BBLs should be fully machinable. It isn't untill they are hardened that reaming would becom a problem. I may be incorrect but having been a machinist for a short while that is how I understand it.
This I do know though, SS will sometimes "work harden" in the blink of an eye. I have seen a knurling wheel destroyed because the machinist student made a return pass with the knurling tool while working a piece of SS. I have also seen a machinist or two totally smoke a drill bit because the feed pressure was to low.
With this in mind I think it is possible for the gunsmith to possibly bear responsibility. One attempt to feed the reamer with the lathe running in reverse while distracted with some other business or conversation may have been all it takes to litterally rub a hard spot (the size of the reamers contact area for that process) in the steel. If the gunsmith fails to notice the problem and get below the hard steel the reamer will fail to cut and will snap or twist.
In short, I would not jump on the BBL maker just yet. And I wouldn't ask the reamer company who is at fault either. Who do you think they are going to blame with the gunsmith buying their reamers.