Thanks, all. I've ordered the carbide expander ball.
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.
Then, while waiting for the 7mm expander ball to arrive, I decided to work on some .30-06 cases. My Redding .30-06 die also makes very straight cases, and I immediately noticed that the expander ball pulls through much more easily than the one in the 7x57.
This reminded me that several years ago, I had a similar problem with my .375 H&H, and sent the die back to Redding with some fired cases. Along the line of GSSP's suggestion, the folks at Redding said the die was sizing the necks more than necessary and reamed it out a little. It still uses the expander ball but works perfectly now. So if the carbide ball doesn't solve the problem, I guess I'll be going that route.
