I was neck sizing in a simple Lee Loader, 36 grains of IMR 3031 behind a 150 gr PSP, still getting seperations.
Did you check your headspace? I use 40 gr of IMR 4895 behind a 150 gr Speer soft point, no problems. I have plenty of brass, but some of them I’ve reloaded 3-4 times. I occasionally get a cracked neck, but no head separations in my reloads . . . I HAVE gotten some head cracks on first firing of some old Greek surplus ammo . . . it’s from around the time of the Cyprus conflict so it’s probably just old and tired. Boxer primed though.