Witnessed an interesting conversation at a GS last week.
Customer came in looking for a neck die saying that his uncle - who was a expert sharpshooter out to 2000 yards said he needed one. GS said they don't have one and recommended a FL die for sizing. Customer said how can that be? His uncle is a sharpshooter out to 2000 yards and said he needed a neck die.
If this was you, the GS did not mention that neck dies size the neck only and don't bump the shoulder back - you'll find that bolt guns are OK with this. Auto loaders on the other hand prefer FL dies for reliable feeding.
Essentlially neck dies preserve most of the dimensions of the fired case, which resemble the chamber that it was fired in, hence more consistent brass, specifically to that gun.

Why not just use the FL dies to neck size. Much easier than having two separate dies.

When you run the case up in the FL die the body of it will be sized before the die sizes the neck of the case.

I didn't think that the body would get resized. The only difference is that your not touching the shoulder of the case. This the way I've been doing for years. If can find the instruction manual that came with my RCBS dies it also goes over on how you can use the the FL dies to neck size.

Put your case in the shell holder, make sure the die is backed all the way out where it just makes contact with the case mouth, raise the handle, screw the die in 1/2 turn and run the case into the die then raise the handle again and inspect the case, you will see a "ring" where the die has made contact with and sized the case neck, keep repeating and you will se the "ring" move down the neck toward the shoulder, make sure to stop before the ring makes contact with the shoulder and lock the die down. You may have to adjust the decap ,sizer stem, to make sure it sizes the inside of the neck on the stroke, and you may want to decap on a seperate step.

did not try that! But wouldn't it size the body a bit?
All these years I bought neck sizing collet dies from Lee.

I believe it may. Not to sure on that. But that's what I have been doing.

I didn't think that the body would get resized. The only difference is that your not touching the shoulder of the case. This the way I've been doing for years. If can find the instruction manual that came with my RCBS dies it also goes over on how you can use the the FL dies to neck size.
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Much of it depends on the chamber of the particular rifle whether to Full or just Neck resize. Brass expands to the chamber dimensions and headspace of the particular rifle. FL Dies squeeze it back to the SAAMI specs. Some fired case bodies may contact the die before the neck gets to contact the die.

There is additional voodoo magic that goes into accurate reloads: Component weighing, flash hole uniforming, neck turning, concentricity gauging, meplat trimming, and small animal sacrifice.

If you want to craft ammo to have optimum accuracy, then you will need purpose built tools and components to do such.

Much of it depends on the chamber of the particular rifle whether to Full or just Neck resize. Brass expands to the chamber dimensions and headspace of the particular rifle. FL Dies squeeze it back to the SAAMI specs. Some fired case bodies may contact the die before the neck gets to contact the die.

There is additional voodoo magic that goes into accurate reloads: Component weighing, flash hole uniforming, neck turning, concentricity gauging, meplat trimming, and small animal sacrifice.

If you want to craft ammo to have optimum accuracy, then you will need purpose built tools and components to do such.

So THAT'S  what I've been missing...