The shoulder is a restriction (just like your thumb over the end of a water hose), the pressure from the case body (expanding gas) hits the internal wall of the shoulder and pushes it forward (like the water in the hose trying to push your thumb off) while it is releasing the bullet, which has resistance to moving (intertia) and the brass is still being pushed forward likely until that projectile exits the case mouth and there is a pressure drop. The brass 'thins' back at the junction of the case wall and case head.
This happens every time the case is resized 'too short' for the 'too long' chamber. The cartridge manufacturers have no control over this 'fit', we as reloaders do, and it is our due diligence to make the best fit possible rather than just continuously full length resizing for a too long chamber. Our reward is 'custom fit brass' to our rifles, which last longer and may be as good, or better, in accuracy and far less expensive.
This thinning is bad enough in typical cases and possibly even worse in rimmed (and a belted case is just a rim in a different place) where the back end is locked in. The belted cases may exhibit case head failure more quickly because they are typically used for high pressure loadings.
Make the rim and shoulder contact their respective 'seats' at the same time in chambering and happiness and long life will be found.
I don't think you have this quite correct
as stated in the article, creating a false shoulder would do NOTHING if what happens is as you say, as the main shoulder area would still create stretching
the shoulder is not moved forward so much as the neck/shoulder junction is moved out an an angle... pressure is not linear, but radially centric. Think balloons.
as soon as pressure builds, the WHOLE case expands, even the neck area to release the bullet, that is why there is gas cutting
as the pressure builds, the front half of the case grips the wall directly outside it, and the front moves out and forward, the rear moves out and back (out as much as it can, as brass thickens, it resists the outward movement.
it is THIS pushing of the base back that causes the thinking near the base. IF the base is held tight on the bold face, there can be NO Thinning near the base to create separation
with a rimmed or belted case with TIGHT headspace there will be NO thinning near the base.
rimless are the WORST for this effect if too short headspace.
MANY MANY wildcats move the shoulder foward and outward with NO case stretching at the base.
With SMLE's in 303Brit, with historically HUGE chambers, what folks have done to create perfect brass, is to expand the neck and create a false shoulder to hold the base TIGHT against the bolt, wrap a 1/4" wide piece of tape to "Fatten" the base to it is chambered concentric, lube the case, and fireform with cream of wheat.
out pops out a perfectly sized case, capable of excellent accuracy if the rifle is up to it.
There is NEVER any thinning near the base.
The thinning of the case head has nothing to do with the sizing. It occurs when the cartridge is fired. The lengthening of the case is a consequence not the cause.
it does! With a rimless case with too short headspace, the firing pin pushes the case forward upon impact, the shoulder hits the front of the chamber, the powder ignites, pressure builds, brass expands and grips the walls, and the base has no support so moves back, stretching at the area between where the brass is gripping the walls, and where brass thickens to the point of not being able to expand.
This only happens with higher pressure cases as proven by Ackley
The belt of a belted case experiences the same problem as the rim on a rimmed case. They are rigid and do not permit the case to conform to the inside of the chamber.
belts and rims do a better job than nothing.... when a case is too short in the HS department, if there is nothing to hold it back, it WILL cause problems
this is well documented and why they sell "Broken Shell Removers"