Hey Dale...where you been? Glad you're back.
As I read it Forming Dies
, it depends on what you are reading and how they are using their dies. There are three die sets available for reloading (incl. auto's and semi's):
1.) small base full length [SBFL]
2.) standard full length [SFL]
3.) neck sizing [NS]
The SBFL die is used to reshape brass to minimum specs so the rounds will chamber and extract with ease in any
automatic. It "works" the brass the most but the ammunition is "pass around" safe. Your brass, when fire formed, does not need to be reshaped to minimum spec if it is intended to be reused in the same AR every time.
The SFL die is used to reshape new and fire-formed brass, and is particularly useful for bolt and single shot rifles. This die can be used to neck size without shoulder set back or to "bump" the shoulder just a little while neck sizing, but the ammunition produced is not for sharing with other automatics as it may neither chamber in nor extract from another. You are correct, the SFL is sizing the whole case, even when you are using it to neck size and shoulder bump.
The NS die does just that, and does not bump the shoulder back, thus increasing fire formed brass life. Again, this ammunition is suspect for any other automatic for the same reason as the SFL. Eventually, ymmv, the case will "grow" in length as the brass flows and a SFL die will become necessary to set the should back a little and to reshape the case...but you don't want to find this out after you have reloaded a few that will need to be unloaded and reshaped in order for them to fit your chamber.
Hope that helps. I like GB's simple and direct advice with a smidgeon of SFL neck sizing only as you reload. You AR's chamber, fire formed brass in that AR, and your regimen of reshaping the brass will tell you what is and isn't prudent. Good luck on your "trek of 1000 actions" to accurize your AR. Keep us posted...