I've come to the conclusion that it's the expander, and here's why. Please correct me if my thinking is flawed!
I have neck sized .300 Win Mag using the same shell holder and can hold .001"-.0015" TIR, so that leads me to think that it's not the shellholder. I can neck size in the .257 die without the expander ball and get .001"-.0015" TIR. I take that same cartridge, force the expander die into it, (liberally coating the inside of the neck with Imperial) and get the familiar .006" TIR. I can partially neck size the brass, rotating it slightly every time I run the brass deeper into the die, but measuring after the final pass still nets me .004" - .006 TIR. Fired brass runs .001" TIR from both guns, both Weatherby and Norma Brass.
I can rotate the shellholder at all "hour" positions on a clock face, neck size with the expander ball out and get .001"-.0015" TIR.
I ordered a new shellholder, "S" type bushing neck sizing die, and .280 thru .284 bushings today. They should be delivered Thursday. I'll fireform via COW the brass that I boogered up to get it straight for the new die. I'll bet I have over 50 pieces of virgin brass to fireform tomorrow.:( I wonder how Kirby's PK brake will like COW?
BB: The shellholder is held pretty tightly with the c-clip ring, and it won't rotate unless I turn it manually.
Guys, your insight into this problem has helped greatly - THANKS from the bottom of my heart. I have learned some new things fighting with this problem, and I will file the info away. I'll let you know how things pan out with the new die.