All cases stretch and can separate over time.... just a matter of degree and the overall case life depends on which type of fatal degradation comes first type thing.
Three things usually in concert. One like you posted is excess
headspace causes more rapid stretching thus more rapid path to separation. Another is
pressure. At a certain pressure, brass begins to flow. It is getting pressed outward against the chamber walls and really the front part of the case, forward of the web, has only one direction to go... forward thus the stretching. The most common area to show that stretching and thinning is the junction of the web and body (the location of the shiny ring) and most common location of separation. One factor sometimes left out of the equation is cartridge design. Seems simple but fairly hard to describe and to some extent a matter of degree. Some designs tend to stretch more than others. Some of reason has to do with amount of headspace control in the design.
Another part of the equation is type and quality of brass... but best to leave that out of this discussion and usually not as great a factor as certainly the first two variables of design, headspace and pressure.
Interesting also is that in response to the
necessary evil of some headspace for function, the case stretches in
two directions. The sequence is usually lost in translation or in a form I often hear as misunderstood mechanics. The cartridge is sitting in the chamber. The headspace control is holding it from moving forward...(+/- the built-in sizing headspace required for basic rifle cycling function)....
mostly but not completely. The firing pin strikes the primer. This is where the misunderstood mechanics begin. The firing pin simply does not have the impetus force available compared to the inertial mass of the cartridge plus the headspace control of the case to cause much if any excess or extra headspace issue. The primer on the other hand does have the energy to overcome that inertia mass of the cartridge and enough to over-run the headspace control of the cartridge. A case for example with a large, sharp shoulder may have enough headspace control to mitigate most of the primer force and the over-run will be minimal. Continuing on with the sequence of firing. The primer fires. The primer "wants" to back out of the case. Depending on mass of cartridge, charge in primer, type and amount of headspace control...
the primer backs out against the bolt face simultaneous to the case moving forward to the headspace control and over-runs that control to some degree. The charge lights and comes up toward full pressure. Momentarily the front part of the case sticks to the chamber walls under the intense pressure. At the same time the head of the case is driven back against the bolt face, reseating the primer and stretching/thinning the case wall just forward of the web. The case depressurizes as the bullet leaves the muzzle and the spring-back in the brass allows the case to ever-so-slightly shrink back from the chamber walls and be extracted. With a normal load, the pressure was not excessive, the primer "looks" normal but the "hidden" stretching/thinning has been done.
If in doubt about how this sequence occurs, load a cartridge to relatively low pressure- maybe on the order of 25-30 kpsi. Seat the bullet deep enough to stay well clear of the lands. Clean all lube off the outside of the case and from within the chamber. Fire the round. If the pressure is enough to momentarily stick the upper case to the chamber walls but lower than required to stretch the case rearward to contact stop against the bolt face, the cumulative amount of headspace will show as a
backed-out primer. That backed-out primer was not caused by firing pin impact force!
So, for example, your cases are sized for .003" headspace in your rifle and your case design allows an additional .oo3+ upon firing. The net cumulative headspace in that instance would be .006". Add to that the pressure component and it all leads to more stretching per shot thus shorter case life and fewer shots until case separation compared to a case that has a more positive headspace control design and is loaded at lower pressure which will stretch at a lesser rate per reload/shot.
