^^^^^^^
You are missing the point a bit.
Yes, partial resizing is a good technique. But you don't have the benefits of two advantages of a collect neck sizer:
A conventional resizer first makes the neck too small then pulls an expander through it. This works the brass a lot. A collect neck sizer squeezes the neck onto a rod. This works the brass a lot less.
Pulling an expander through a neck often makes it crooked. Collect neck sizers often make straighter ammunition.
Most find that after some number of loadings with a collect neck sizer, they still need to bump the shoulder. Even if you only partially resize on these occasions, you work the neck in ways you would prefer not to do.
A body die allows you to set the shoulder back when needed without touching the neck. You can often make a body die from a FL sizing die.
That is the biggest problem. It overworks the brass and then the expander ball becomes the patsy for runout because of the extra work it has to do. This often occurs even when the expander ball assembly is perfectly straight.
A particular cartridge I have a lot of experience with is the 308 Winchester. Neck wall thickness varies quite a bit for the 308. WW brass has typically been .012" thick. Double that plus .308" and you get a loaded neck OD of .332". So the neck needs to be sized down to .330" or so, and most off the shelf FL sizers go smaller than that. I have a Redding FL sizer that does .328". Its expander assembly is straight, and with the slick carbide expander ball installed it sizes WW brass pretty straight and the necks come out .330".
Now let's load that Lapua brass. Necks on it have typically been .015" thick. Twice that plus .308" is .338", so the necks ought to be sized to .336". But the Redding die squeezes them down to .328", and then the expander ball needs to open them up .008". So the brass got cold worked an extra .016", that is, .008" down too much and then .008" back up. On top of that the runout isn't so good and so the groups on target suffer. The handloader who doesn't understand the mechanics of the situation might say that fancy Lapua brass is a ripoff.
