Case hardening - exactly.
Annealing “softens” the metal so it can withstand the stretching that takes place when the case expands to fit the walls of the chamber.
If you don’t anneal the metal will become hard and will develop cracks sooner.
Whether on not there’s a shoulder on the case isn’t the issue, it’s the pressure that the case has to handle that is.
How much the metal moves is also a factor.
Straight walled cases are worked less than bottle neck cases. The bottle neck causes more of a stretch on the case because the shoulder allows the pressure to be applied longitudinally. A straight walled case has nothing for that pressure to directly work upon.
two things harden and wear straight wall cases. 1) Firing… detonation, expansion and contraction. 2) Resizing. eg. compaction.
With low pressure cases (eg 25ACP), powder detonation is like being tapped with a jeweler’s hammer. With high pressure (eg 500 S&W), it’s a sledge hammer.
Thinking out loud here.
The amount of crimp might play a role. Bottlenecks are not usually crimped much, if at all. Heavier magnum straight walled cases are often crimped quite a bit. That would create a lot of movement right along the edge where cracks form
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