Chick, I've seen a couple of caseheads from ammo used in Garands that's been full length size "just like the manual said to have the die hard against the shell holder with the ram at the top of its stroke" instructions, or the equavalent there of.
Here's what happens with rimless bottleneck cases when they're used in M1 and M14 rifles:
1. Round's chambered and the bolt closes, ejector's very strong spring pushes the round full forward in the chamber with its shoulder firmly centered in the chamber shoulder.
2. Firing pin's driven hard into the primer with enough force to push the round a bit further into the chamber setting the shoulder back a thousandth or so, then the round fires.
3. Chamber pressure builds up pressing the thinner case walls right behind the shoulder hard against the chamber as the bullet gets pushed out and into the rifling and the primer gets pushed out of its pocket a few thousandths.
4. More pressure builds and presses the case head back against the bolt face stretching the case's back half as more of the case body presses against the back part of the chamber wall. The primer's now pressed back into its pocket flush with the case head and the case shoulder's hard against the chamber shoulder; the bullet's now about 40% down the barrel.
5. Bullet goes past the gas port then exits, gas pressure in the cylinder pushes the op rod back opening the bolt and extracting the case from the chamber.
That fired case now has a greater distance between its head and its shoulder than when it was loaded.
Full length sizing that case and setting its shoulder back too far, reloading it then shooting it again in that chamber repeats the above. But the case gets stretched again and again each and every time it's reloaded. That stretches the brass about 1/4 inch up from the case head too much; it finally separates on the case's last firing when that "web" is the thinnest and weakest.
Sometimes, you can feel the inside of the case have a "groove" at that point with a long wire with a hook on its end moved back and forth inside the case at that point. If you can feel it, that's called incipient head separation; it's gonna happen in the next 1, 2, or 3 reloadings that set the fired case shoulder back too far each time.
US Navy Distinguished Marksman Badge 153
Former US Navy & Palma Rifle Team Member
NRA High Power Master & Long Range High Master
NRA Smallbore Prone Master