The description of the shoulder changing shape bothers me. It can be expected to become a bit bigger, but the shoulder rounding should not occur. I took a look at the Reamer dimensions on Clymer's web site, and there is no shoulder rounding indicated. I also noticed they make both a standard and a match reamer for .17 HMR. The match reamer has 3.5 mils smaller base diameter, 2.5 mils smaller shoulder diameter, and it is 0.5 mils smaller in the case neck. Rather than shorten the freebore as most match reamers do, they increased the throat angle from 1.5 degrees to 2.5 degrees to get the bullet started on the rifling sooner. I don't know why they chose that alternative? Getting a chamber cut with this match reamer might reduce the case expansion just enough to achieve case survival.
Any chance you could post a picture of a fresh cartridge adjacent to one of your fired cases that did not split and one that did?
One other thought: Next time you go to the range, place all rounds in the chamber individually using your fingers so you can put the headstamp lettering in the same orientation for each shot. This way you can see whether the split occurs at the same place in the chamber each time? If so, it points to a chamber defect; being slightly oval, for example. This might explain the shoulder rounding if it appears more on one axis than the other?
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