Re: neck bushing die v. neck die?
The RCBS neck sizing die does not resize the body/shoulder of a case only the neck. It reduces the neck diameter beyond what is needed then the neck is pulled over a sizer ball to create neck tension. Most of the RCBS dies I that have measured use a sizer ball that is .003" less than bullet diameter. This die is made to accommodate different brands of brass with variations in neck wall thickness which is the same approach with RCBS full length sizing dies.
A bushing die also doesn't size the body/shoulder. There is no over sizing of the neck like the RCBS neck sizer die. There is no sizer ball needed to open up the neck, which could cause excessive runout of necks. Instead an interchangeable metal tube called a bushing is placed into the top of the die to reduce the neck diameter a specific amount. Bushings are made in .001" increments which makes it easy to adjust neck tension for the brass you decide to use.
Hope this helps.