Case head stretching isn't assumed. It is evidenced by the thinning of the pressure ring. That proves the case was stuck and the sides held forward as the head backed up.
Unclenick, 99% of the time I'm right there with you on things, but not this time...
What I see are results of firing. How the case got into that condition is where I differ with dogma.
Yes, the case stretches & thins, mostly just above (or in front of) the head/web.
The current dogma is the firing pin drives case forward,
Front of case bloats/sticks first,
The back of case moves rearward stretching the case.
I don't see it happening that way.
What I see is bullet movement pushing case backwards at the first couple hundred pounds of pressure, what ever it takes to dislodge the bullet from the case.
The pressure the primer produces can easily squib a bullet out of the case into the barrel...
When bullet dislodges, the case MUST move backwards.
The primer is indented, even if firing pin is fixed, the dent in the primer will seat right down around the extended firing pin, and the case hits the bolt face...
Now, anyone that's ever left powder, particularly rifle powder on a painted bench top, or on a piece of paper knows you have a stain.
The ONLY way you can have a stain is something running out of the powder... That means an non-compressible liquid.
The pressure starts in the back of the case with the primer, the bullet unseats overcoming any forward movement... The primer dimpled sits right back down over the firing pin, no matter if it's retractable or not.
The burn starts in earnest in the rear of the case and throws NON-BURNING powder forward against the shoulder & bullet which is now in the throat.
Bullet moves forward, case moves rearward with enough force to overcome firing pin (dented already), and the few pounds of ejector spring force, assuming there even is a spring loaded ejector...
The case is fully rearward pressure wave moving forward compressing powder that has SOME moisture in it (again, can't leak/stain or vapor off attacking plastic powder bins if it's not VOC),
That same moisture is why powder clumps when compressed.
That same moisture becomes a hydraulic hammer against the bullet & case shoulder.
At this point, the HOTTEST part of the case stretches, and the THINNEST part of the 'Hot' stretches the most.
That would be just above the head/web.
The thicker case head works as thermal mass, a heat sink, the case sides back where the powder is burning hottest is thinnest spot exposed to that heat, it's what stretches when the pressure expansion wave hammers the case neck (and back of the bullet sizing it in the rifling).
I could be wrong, but it fits the physics better than the shoulder/neck of the case (little surface area & extreme reverse angle for gripping anything) holding the case forward, even with gas jet venting forward, combined with the mass of the bullet exiting the case long before there is enough pressure to expand the case...