As 45s r best mentioned, a spent case fired in a normal chamber will have a flush seated primer because while the primer backs out as much as case length and headspace allow, it gets seated flush when pressure causes the case head to contact the breech face. Based on SAAMI specs, a minimum length loaded case (0.744") fired in a maximum headspace chamber (0.776) will see the primer back out 0.032". But the fact that the primer didn't get reseated seems to indicate the pressure didn't get very high - at least not high enough to reseat the primer.
The separated case would explain the lower pressure, but why it separated in the first place may never be known.
However, if the case is still available the average OD of its "mouth" - the average of its maximum and minimum OD's - will give a little idea of the ID of the chamber it was fired in. This OD can also be compared to the case OD just at the forward edge of the extraction groove to get a little idea of how high the pressure reached.
When you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind. [Lord Kelvin]
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