I think that maybe we're looking at the idea of case head expansion wrong. I have visited with a few reloaders who do hotter than normal loading. The type of over pressure loading that many guys say gives you a three shot case life.
The guys that did the hotter than normal loading said that they sized their brass a few thou bigger than the chamber so that the bolt is actually compressing the case when the handle is turned down the whole way. They have all told me that the case life was very acceptable doing it that way. 10+ shots per case.
Later I did an experiment with full length resizing a 270 winchester. I started with brand new winchester cases. I lubed the brass after seating the bullet with a very light film of lithium grease. And I ran a load pushing the 150 gr bullet out of the gun at 3050 fps according to the chronograph. My cases didn't show any loosening of the primer pockets after 10 loads each. Neither did they show case head separation, or case head expansion. I even used one case that came from the factory with a loose primer pocket, and the primer pocket never got any looser, and neither did the primer flatten more than the other cases with good pockets. And I did measure with a micrometer. The difference was negligible from the first shot to the last.
This has led me to the theory that case head expansion is caused by case head flex caused by the case walls gripping the side of the chamber, the pressure pushes the primer back against the bolt face; then the pressure bows the case head backwards, center first, (the weakest point) pushing the primer back into the pocket, flattening the primer; then the web of the case is stretched after the case head pushes against the bolt face, reflattening the case head against the bolt face. The stretching of the case web can gradually lead to case head failure if the cases aren't taken out of use soon enough.
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