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That’s a heap of extra and un-necessary sizing taking place and is certainly contributing to, if not the cause of your necks splitting.
Bushing dies with a .314 bushing would half the amount you are...

Are you using bushing dies?
If so, are you just sizing .001-.002 under .315 as any smaller will be over working and hardening your necks.
If they are standard dies then remove the expander and see...