I have a Mitutoyo blade mic. I bought it a long time ago when playing with a Oehler M43 Ballistics Lab.
The other thing which comes into play with brass is that when it yields stress vs strain becomes distinctly non-proportional - there's a sudden increase in plastic deformation without increase in applied stress in the area of the stress/strain curve just past the yield.
Another good point. But...what comes first? CHE that is indicating overpressure or the obvious signs of over pressure as in ejector marks and flattened primer?
This is another reason why we don't see a linear relationship between chamber pressure and CHE. What instead you have is, as I said earlier, a much simpler set of criteria: if there's some small but measurable amount of CHE, then you are pushing it. If it is enough to do things like make primer pockets loose and/or cause plastic flow into the ejector then that is well over what that particular lot of brass should be expected to take.
Are you saying that measuring a fired case (not a brand new case but one that has already been fired one or more times) should no longer show any expansion above the web?
