I would like to see this same test done with fire formed brass and shot in a pressure barrel. I've seen some comparison work done with a pressure barrel and what the primers looked like, it was an eye-opener.
The parent case was rated for how much pressure? The web is the determining factor when the operation pressure is rated.
The accepted method of determining pressure of a belted magnum is to measure the head spacing belt before firing and after to find the amount of expansion. There are differences in expansion between the manufacturers with the same load. You can find the resulting opinions on that with your search engine.
