Wow.
@Clay McGill took this thread totally off the rails! Anyone reading this could easily get confused and since this type of failure seems to show up every so often this might be a good time to explain what really happens to brass on firing and on resizing so I'll take a shot at it. Just went through this on a different forum.
First off, let's make it clear that this failure is not caused by overpressure although pressure does play a very small part on when it was going to occur.
The cartridge cases have several functions. It holds powder. It holds primer and bullet. A portion of the case is a pressure vessel (this is the unsupported portion from case head to the chamber, it serves as gasket material between the unsupported portion and the barrel to prevent high pressure hot gas from escaping from the chamber.
To allow chambering the cartridge must have clearance axial and longitudinally to fit and allow the bolt/breech to close. The longitudinal clearance is headspace clearance and is the dimension of interest. It is not the headspace dimension shown the SAAMI chamber drawing. It is the difference between the chamber headspace dimension and the cartridge datum dimension for the shoulder.
On firing the cartridge body, shoulder and neck begin to expand axially to fill the chamber and longitudinally to fill the headspace. as the body expands the shoulder and neck towards the the head and as the body contacts the chamber wall the pressure rises and begins to form a new shoulder from the old shoulder and neck. Ultimately the case and barrel expand axial and longitudinally together under the full pressure of the powder charge. As the pressure decays the barrel and brass contract with the barrel returning to its original size and the brass just a slight bit less (at normal firing pressures) than the barrel. After firing the case body will be larger in diameter and the datum dimension of the case will be longer than the original case. The overall length of the case will normally be shorter. The overall volume of the brass will be greater and for this to happen the the case must become thinner somewhere. This typically offers in the thinnest portion of the body.
When the fired case is resized in a full length sizing die the first thing that happens is the body begins to be squeezed back towards the SAAMI max case diameters. This squeezing causes the datum dimension to increase, beyond what the chamber can accept so the shoulder portion of the die then reshapes the shoulder and the body to forma new shoulder of the correct datum dimension and some of the original shoulder ends up in a lengthened neck. Ultimately the case length increases. In some cases the expander ball lengthens the case and will require excessive setback of the shoulder and ultimately some stretching of the body of the case.
As for over pressure (say 55KSI to 75KSI) the ultimate pressure containment is the chamber and the case head. The axial growth of the cartridge is restrained by the receiver/barrel and the bolt lugs. The additional pressure in the high strength reciever, barrel and bolt does not allow for a significant amount of additional growth but there is additional growth.
The reloader has little control in brass movement due to the body resizing other than by minimizing differences in chamber/die dimensions but he does have control over the amount of headspace clearance on firing and excessive clearance leads to excessive growth and ultimately enough thinning to lead to failure as shown on the original post. Lower pressure loading might have allowed for a few more firings but the brass would have failed at the same way at some point if sized the same way each time.
Hopefully this will help with understanding what we've been trying to explain about how the case sizing caused the failure of the OP's case.
