"I also tried the expander ball from my Redding .280 Rem. FL die, which makes very straight cases. It pulled cases just as crooked as the one that came with the 7x57 die."
That would make me think that you might have cases with uneven neck wall thickness, because by switching the expander ball, you have eliminated any out-of-roundness.
It could also mean that the die is out of round or off-center where it is drilled and tapped for the de-capping rod.
If you only substituted the ball from the .280 die, and not the entire stem, the problem could be with an off center stem.
Try the entire stem and re-sizer ball from the .280 die and see if the cases come out straight.
I am not very familiar with Redding dies, having only owned one set. My experience is with RCBS dies, and I am assuming that the Redding and RCBS dies and stem operate about the same way. I did not say interchangable, but operate.
