The shoulder is the weakest part of the brass. Some of the brass works up out of the shoulder and into the neck. The expander ball is pulled back through the neck, and when it is, the shoulder is not supported. The expander by it's nature works the brass. If the expander is not perfectly centered(which it rarely is) it pulls off to one side or the other. This pullingis what yields bad run out on the neck. This is also a reason why a lot of people trim the necks, even if they do not have a tight chamber. The more consistent the neck brass thickness, the less sizing will cause run out.

Also a lot of people do not clean the inside of the neck before drawing the expander ball through the neck. This IMO is also a mistake. Dirt can build up in the neck, and on the expander ball. Both will cause a negative effect. A lot of reloaders will polish the expander ball when the get new dies. The polished expander ball will pull more smoothly through the neck, which will have less effect on the neck.

I always remove the expander ball out of my dies. I use the Sinclair neck mandrel. And I only size cleaned brass. It takes an extra step or two. But then I enjoy my reloading. So it is no big deal to me.

I guess you just have to keep in mind that the neck holds the bullet. So if you pull the neck off center just .001, that will have a compounding effect at the end of the bullet. And if the neck is not concentric with the case, well I think you get my point. Tom.