To much stretching and you get a separation.
Have a gunsmith check headspace, or keep shooting factory new ammo.
And then reloaders need to decide what happens to the case when fired, most believe the shoulder moves forward, if the shoulder moves forward I will assure the case has no choice but to stretch between the case head and case body.
And then there is the reloader; the reloader insist on sizing the case back to minimum length/full length size; problem, the reloader does not have a SAAMI factory minimum length and minimum diameter chamber. Once the case is fired and it is formed to the chamber stop sizing; I am the fan of cutting down on all that case travel. I am the fan of forming first and then firing.
One more time; I purchases a Model 70 Winchester 300 Win. Mag. The rifle had the ugliest chamber I have ever seen; the chamber was too long and the chamber was too large in diameter so I called Winchester, to them it was an easy fix, all they had to do was polish, ream and or hone. They could not tell me which of there three options would make the chamber shorter and smaller in diameter. They contacted the local warranty smith, a man I know and gave him instructions; he had the rifle for 2 months and then returned the rifle to Winchester. He said they sent it back because the chamber was too large and then I asked; "When did the chamber get too large, was it after you honed it, or polished it and or reamed it? That was when he informed me the chamber was too large when I brought it in.
Months went by; I called Winchester and tried t make them a deal, I wanted a chamber in that rifle that matched my dies or I wanted Winchester to make a set of dies that would match their chamber. Spring back? The fired cases had no memory of ever having been a 300 Winchester Mag. case. There was no way to stuff those cases back into a die, I have forming dies, I have BAR 300 Win, Mag. dies and I have small base dies for the 300 Win Mag. case.
Anyhow they returned the rifle in a new box, that had to be 14+ years ago; I dug it out 2 months ago thinking I would take it to the range with 100 rounds and then changed my mind. The bolt would not close on factory, new over the counter ammo or my reloads. I fixed-it and gave the ammo to someone that does not reload.
F. Guffey