Below is my Redding FL .243 die equipped with a modified Forster expander and spindle assembly. I will never understand why Redding uses a oblong expander that creates so much drag even if the inside of the neck is lubed. The Redding expander induced far too much neck runout and now with the Forster expander the runout is .001 or less with neck turned brass. NOTE, when the necks are not turned your neck runout should be the same as the neck thickness variations.
My preference in dies are Forster full length benchrest dies with the high mounted floating expander. The Forster dies produce the least amount of neck runout out of any other die I have ever used. Bottom line, I recommend anyone to try a Forster benchrest die and see how concentric your cases are after sizing.
Below the expander enters the case neck while the neck is held and centered in the die. This design prevents the expander from pulling the neck off center and inducing runout.
I would still like to know "WHY" the Redding die failed to push the shoulder back far enough. I would also contact Redding and see if they will fix the die or give you a new one.
I use Redding competition shell holders that do not push the case into the die as far. Meaning the five different shell holders control the amount of shoulder bump in .002 increments. I have a Lee FL .223 that will push the case shoulder back .009 shorter than a GO gauge. And most dies will push the shoulder back far more than necessary and it is very rare that a die fails to push the shoulder back enough to chamber. Nothing is written in stone but only once in 47 years of reloading did I need to lap the top of a shell holder to get enough shoulder bump.
When you full length resize the case it grows in length as it is squeezed down in diameter. And it is the last few thousandths of sizing that pushes the case shoulder back. And a small base die will cause the shoulder to be moved further forward before being pushed back and need to be trimmed more than a standard die. Normally if the die fails to push the shoulder back far enough you have tight or shorter chamber headspace and a "long" die.
In the Savage forums you see this problem all the time when the barrel is changed and the headspace is set too tight using scotch tape on a new case as a headspace gauge. And their next posting is asking why their die isn't bumping the shoulder back far enough.