I agree with Mooseman to try different ammo, likely the ammo is the problem if every single one split.
That happened to me with a particular batch of 1942 .303 ammo. Of the fifty in that bandolier every single neck split. That said, they were also fired in my most generous chamber sized of my SMLE's, might not have split in a tighter toleranced chamber.throat. But even that larger chamber does not routinely cause split necks, so back to the ammo again.
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