Here are some "post mortem" photos of various cases. Sorry about the poor quality, I had to cut the resolution down in order to be able to upload them.
The first is a comparison of hornady's brass to starline's, specifically the rim to web area in the head. Hornady's chamfer from the main case down to rim groove is noticeably longer/deeper. Is this significant? I don't know; I can't measure the degrees of the chamfer though I'm guessing hornady's is the lesser of two.
In the next photo I have of once-fired starline brass used in an early test of the 180 hot core .357--even when the cartridge fired successfully there was a noticeable compression of the distance between the rim face an top of the head/web. leaked gas was also evident in the area. The blue lines I added to exaggerate the compression effect. The area in both the intact case as well as the blown one that are compressed and have gas accumulation appear to be roughly the same size as the width of the extractor.
In the last photo of the blown case you can see the area of failure is also compressed and about the same size as the compression area in the fired case that did not fail. Notice also the concave slope of the over-expanded case head area where it meets the case main body--I believe that is analogous to the radius machined into the chamber face.
