On page two of the thread Josiah Thompson asked Thomas H. Purvisabout that chamber mark in the thread that Lance provided the link to. Here’s part of his answer:
Since CE543 did not show indications of the dent and the live round did, then this tended to serve as further circumstantial evidence that CE543 came from the first shot fired in the assassination shot sequence.
Thomas provides what seems to me to be a reasonable answer READ THE THREAD.
https://educationforum.ipbhost.com/topic/9276-an-unfired-cartridge/page/2/
Yes, Purvis seemed quite knowledgeable to me, although the late Robert Prudhomme dismissed his analysis as "bull crap." As I understood what Purvis was saying, the chamber marks (or lack thereof) would vary according to the thermal conditions in the chamber, with the first shot obviously being in the "coolest" chamber and thus the least likely to be marked by the chamber wall. I don't want to pretend to more expertise than I have (pretty much none!), but I did look at several online forensic discussions and manuals and they seemed consistent with what Purvis was saying. I would logically think that the possibility of reload ammunition being used and/or the quality of the rifle might also be significant.
FWIW, here is Michael Griffith's piece on CE 543, which quotes Thompson on the "chamber marks" issue:
https://maryferrell.org/archive/essays/mgriffith/dent.pdfAs you seem to be, I am always skeptical of arguments that purport to be CONCLUSIVE when there are so many potential variables. This is true of both LN and CT arguments.
Six Seconds in Dallas had been published more than ten years before the HSCA firearms panel examined CE 543, but they do not seem to have noted the chamber marks issue at all (they did discuss extractor marks, which are different).