The key words above are "neck turning" and then not needing to use a expander.
And if you neck turn the case neck is going to expand that much more when fired. And then it will need to be reduced in diameter even more using a bushing die. And if you neck turn then you have to deal with downuts as the brass flows upward into the case neck.
And at the Whidden custom die website they tell you they get more concentric resized cases with non-bushing full length dies.
Below is a Redding bushing die cutaway, now look at the clearance on the sides of the bushing and die body. This bushing side clearance is how much the bushing can move from side to side and tilt when sizing the neck and inducing neck runout. And the red arrow shows the area not sized by the bushing.
Tech Line & Tips (FAQs)
Concentricity Problems
https://www.redding-reloading.com/tech-line-a-tips-faqs/146-concentricity-problems
We have conducted many tests over the years on the various factors contributing to concentricity problems with bottleneck cases. We have repeatedly found a definite correlation between the uniformity of the brass (or lack of it) and the resulting concentricity of the neck to the body of the case.
An interesting experiment also revealed that neck turning of brass that was intentionally sorted as non-uniform, showed little or no concentricity improvement when used in standard S.A.A.M.I. spec chambers. Conversely brass that was sorted and selected for uniformity remained uniform and concentric with or without a neck turning operation.
Question: Do I need to use the expander button that came with my Type "S" Die?
Answer: It is advisable to use an expander button to maintain consistent neck tension if the case necks have not been turned to a uniform wall thickness. However, the expander button can be replaced with the Decapping Pin Retainer (included with your Type "S" Die) if the user does not wish to use an expander button.
If the neck wall thickness varies more than 0.002", it may be necessary to use a bushing a couple of thousandths smaller than your calculations indicate, and then use a size button in the die to determine the final inside neck diameter.
My words
, just because you buy an expensive bushing die doesn't mean you will have more concentric cases and make better ammo.