I think the "partial neck sizing" and the "neck sizing" presupposes some consistency in neck wall thickness- within one round and round to round. Back in the Day, I had a hand operated neck trimmer - worked away at some 338 Win Mag R-P brass to get the necks even - and with my store-bought Winchester rifle, I could not show any difference on targets over my other loads. So, I gave up on "peeling" neck walls for that rifle. It might have made me "feel good" - but I could not show any difference on target.
Mostly today I focus on slick chambering for my hunting rounds - there does occur occasions where a follow-up shot is wanted "right now" - that is not a time to find out that I was too "cute" with that round.
At the time, I read that most brands of ordinary dies scrunch the necks to be too small - by pressing on the outside of the case neck wall - then the "expander ball" opens up that neck hole (from the inside) to proper size to seat that bullet - regardless of neck wall thickness. All the holes should be the same size. I even read of one fellow who would size his necks, without the expander installed, then re-install the expander and open those necks by inserting the expander INTO the case - he claimed he had much better consistency (less runout) when he did that.
