I learned this answer from folks on this board. "Did you measure before resizing? If so it is not unusual for some brass to flow into the head and web or sides of the brass, than into the neck. When you resize it will make the brass longer in the neck and thus need a trim. The more you have to trim, the thinner some area of the brass becomes and eventually a failure will occur."
As Chief alluded to, it is not necessarily "firing" a case that lengthens it, but resizing it. Checking a case before it is fired and then again after it is resized will give you some information about what kind of case life to expect.
If you fire a really low pressure load, below around 30,000 psi, you will often find cases are short even after resizing. That is because the brass is not sticking to the chamber walls and stretching back at the pressure ring where the walls join the head, but rather is backing up in its entirety and blowing the shoulder forward, which pulls brass from the neck. This is like what a light fireforming load does.
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