The neck shoulder area of the brass is the most vulnerable to change. Expanding with the use of a ball or a push in expanded can do more harm than good. The case is not supported with the die . Consistent neck tension is done with neck thickness and elasticity of the brass - And or annealing.
Larry
You make some good points. True, the softer neck is easier to distort than the case head and the kind of mandrel most of us use doesn't support the case. However, it should be remembered that the process is in no way comparable to the kind of change we make to a case when we size the neck or bump the shoulder.
In my situation at least, the neck size die is carefully selected to provide a neck size only very slightly smaller than the mandrel. This happens early on in the process. Later, just before seating, the mandrel gets inserted with very little resistance because the neck is expanded so slightly that you can barely feel the process even without any lube.
The process might be better called "uniforming" because it takes care of any out-of-round condition caused by handling the brass during cleaning and other batch processing. It also tends to uniform the ID which will be very slightly different between a case with a neck thickness of .01200 when compared to a case with a neck thickness of .01213 even though both have been processed with the same neck sizing bushing. I neck turn all my brass for uniformity, but that doesn't make them PERFECTLY uniform despite the fact that my 21st Century lathe is a pretty nice piece of equipment.
If constant neck tension is the goal, then it might be obtainable if all my brass has the exact same neck thickness, the exact same hardness after annealing, the exact number of sizing cycles, and if they get gently handled during the reloading process. I try to perform high quality reloading, but I don't pretend to be perfect at it especially the "gentle handling" part since I don't like to treat my cases as though they're made from nitro glycerin. That's why I think inserting a uniforming mandrel down the neck just before seating is a worthwhile step, at least for me.
It is a very subtle process intended only to iron out any tiny inconsistencies between one case an another. It is NOT the same thing as something that might be called "inside-out-neck-sizing".
The mandrel in my die floats and I can't detect any difference in concentrically caused by using it.
But the proof of the pudding is in the eating (as a few people might say). My testing has shown a slight but measurable improvement in Mean Radius and an even more significant improvement in ES and SD since I've been using the uniforming mandrel. Plus I'm now winning a good share of 600yd F/TR and BR matches; something I wasn't enjoying in my pre-mandrel days. Or perhaps it's because of my lucky underwear.
