The diameter at the shoulder is still reduced. You can not neck size with any full length sizing die. The taper of the case prevents that. You can partially size the neck and case with a full length die..
One problem that can occur when you try and neck size with a full length die by backing it off a bit, is sometimes when the neck and shoulder diameter are being sized, the shoulder itself will bulge forward ever so slightly. When this happens the case will feel a bit hard to close the bolt on... This almost microscopic bulging of the shoulder does not occur when using a neck sizing die which sizes only the neck diameter.
There is nothing wrong with full length sizing providing you do not push the shoulder back excessively from it's fire formed form.
Is from one of the recent BR central discussions on the same thing.
Now if the top BR shooters are FL sizing with extreme accuracy, what makes a partial NS or NS only all the time so desirable.
You are still resizing the body some on a FL die that is just unscrewed anway and possibly making it unchamberable.
Now what does this gain vs what does it risk in reality?
Obviously we are talking factory chamber which means factory barrel, so match grade accuracy is not a real consideration and no NS only or partical NS brass is going to give it.
Jury rigs normally give jury rigged results at the worst times. Either do it right with a proper sized FL die and properly adjusted or NS and then use a body die every 2-3x.
Gain is in reality more imaginary feel good vs anything actual. Yet you risk brass that will not smoothly chamber. Which IMO for hunting gun is not smart.
Unless you are using a shoulder bump gauge or Hornady shoulder comparator you do not have a clue how much you are bumping the shoulder back or where it even is in relationship to your chamber.
Now if you go about it slow and use brass that is reached the point of hard to chamber, you can use lamp/candle black and set the FL die up high. Slowly lower it 1/8 turn at a time until it just touches the shoulderand the case will smoothly chamber. That gives you the shoulder bump with minimum brass working and max case life.
Starting off adjusting your die to touch the shellholder is the directions that come with it, but not conducive for minimizing working the brass. You often times are pushing the shoulder back way too much. It will chamber easily and that is why the die mftr tells you to do it that way.
If you want to be that precise and use a factory die in a factory chamber, then do it right. Send your die to be honed to match your chamber by Jim Carstenson at JLC Precision. He can do necks and bodies. That way it will match your chamber.
A properly sized FL die does not overwork the brass and ensures consist neck tension and body diameter.
Now to further throw water on this, the necks are going to work harden no matter what you do and you are going to get inconsistent neck tension after 3-6 firings unless you anneal. No way around that. More groups are lost for neck tension issue than brass bodies overworked.