Originally Posted by MMERSS
Try annealing. You may have overworked brass needing heat treatment.
Under normal circumstances, I would agree that annealing would be the remedy for split necks. The way the OP reads, it sounds like factory ammo is showing split necks after firing? Or do the necks split when reloaded after one firing? In either case, something is out of whack, annealing should not be necessary after one firing ...