I have posted this printed pic I did in paint before showing the internal rut with the tool for feeling it. It has a couple of "feelers" laid on it. Some people like to shoot brass until it shows the bright ring on the outside, and some just shoot it until a crack happens. I would say that is firing the case too many times, but some folks do it. Case head separation happens when we fire a case over and over and when it has room in the chamber to stretch. If a rifle has excessive mechanical head space a case can do this on one firing. Another way is if we push the shoulder back way to far and create "artificial" headspace, or excess clearance in the chamber. It adds up to the same thing. The case has too far to stretch and it thins near the case head. When we fire a bottle necked rifle round the firing pin pushes the round forward until the shoulder stops the round in the chamber. Then the pressure blows out the case to fill the chamber. The softer front of the case grips the chamber walls sealing off the hot high pressure gases from blowing out of the rear of the chamber. The harder thicker case head holds pressure in the rear. This is most important job of the brass, to contain the gases/pressure and hold together during firing. The brass case, with help from the chamber walls and breech face, seals the open rear of the chamber, and forces all the gas forward down the barrel. When fired, the softer front of the case is gripping the chamber walls, while the harder rear of the case which expands minimally is free the move rearward to the breech. This cause stretching of the case, normally right where the thicker web transitions into the thinner wall. It stretches because the front of the case holds tight to the chamber wall while the rear does not. It slides to the rear until the breech face stops it. This is why we do not want to over size, or push the shoulder back too far, on our cases, as it will increase how much the case stretches each time and lead to failure sooner than necessary. We can use a case gauge like those from Wilson, or various tools to measure how much we move the shoulder. Ideally we want to move the shoulder .003 or less. This is a case with a shiny ring. It showed up as a dark line in the pic, but to the naked eye it is a bright line, the one we always talk about. Here is the same case cut open to show the internal rut we are feeling for. Here is another case he shot until it split, almost in half. Cutting it too close for me. Here is that case cut open to show the rut. You can also tell where it is split all the way through on the right side. Here is a pic with a feeler tool shown inside. This case head almost separated. Bad Ju Ju.