I have a Redding FL die in 257 Wby that does the same thing. Redding sent a decap assembly with a diff dia exp, but it didn't help a bit. Same as you say, necks with concentric thickness, but terrible RO after the exp pulls through. Lube didn't help a bit. I put a 25-06 exp in a 270Wby die and use it to expand after I size with the FL 257Wby die. They come out straight this way. Redding asked me to send the die back for inspection. I have a feeling the tapped portion of the die is the culprit.
You'll run into a dud die once in a while. I've had 3 RCBS dies and one Forster that gave terrible RO as well despite adjustments.
It's hard to beat a Lee collet and Redding body die combo for straight cases.
loder
