Full length sizing resizes the whole cartridge back to approximately it original size. This is done so your reloaded cartridge case will be to factory specs to run through any chamber. So it does move the shoulder back if the shoulder expanded in your chamber. I own two .270's so I have to full length resize and trim cases to make my loads work in either gun.
Now I only own 1 30-06 so I only neck size it for about three reloads. That way I am only getting the neck back to approximately it's original size to seat the bullet. Then I will run the case through a full length die. The down side is my reloads may not load in another chamber because it has been formed to my chamber. The upside is I get more reloads because I am not stressing the whole case every time I reload.
