Originally Posted by boomtube
Normal neck sizing below the neck's contact with the seated bullet is meaningless.
Except that it does center the case neck more precicely on the case shoulder. Not much, but noticeable if you use the case shoulder as the front refernce when spinning cases/rounds for neck/bullet runout.
Which is why some accuracy afficianados lap the neck out of a standard full length sizing die instead of using a full length bushing die.
Originally Posted by AZShooter
The feeler gauge will set the die very close if it isn't perfect.
That's right, and lots of folks do that. But there's still the issue of controlling fired case shoulder setback. Two things make the spread of resized case headspace vary.
One's the lubricity of the case lube and how much is put on the case. The more slippery the case is going up into the full length sizing die, the less the press will spring and the case shoulder will be set back more.
The other's how long the ram's kept at the top of its stroke. I've set a dial indicator on the press and watched it slowly move the die down sizing the case a tiny bit more until it stops.
Both of these will easily cause a few thousandths spread in sized case headspace.