An old post of mine:
I've done some handloading for a friend's Browning Low Wall in 260 Remington, also using a vanilla RCBS two die set. As luck would have it the sizing die and rifle chamber are well matched in the important dimensions. Furthermore the neck section of the die is perfect for the batch of brass I've been using. It squeezes the neck down just enough so the expander ball just kisses it on the way out. So simple works great here.
OTOH, I have a Redding FL size die for 308 Winchester whose neck section is quite tight. If I size a piece of brass w/o the expander in place the neck can come out below .330" in diameter. (This dimension can vary depending on the amount of resistance/springback the brass provides because of thickness and state of work hardening.) For typical WW brass this is all right, but the batches of Lapua brass I have only need sizing to .336" for the neck tension I want. So this brass is being worked .012" more than necessary on the sizing stroke, and even with a carbide ball and good lube in the case necks the resulting runout isn't so good. If I size them with this die w/o the ball, and expand using a tapered mandrel in a subsequent step, then the runout is fine. (Even so, the brass is still being worked more than I like so I use other dies for Lapua.)
