Shoulder set back resolved
While I still don't understand exactly how the seating die design can allow the shoulder to be pushed back, my issue seems to have been caused by:
(a) work-hardened cases producing too much "spring back" during sizing
(b) This resulted in poor neck tension, with some bullets held so loosely that they could be pushed back in the case mouth by hand
(c) My attempts to "crimp" the case mouth during seating by adjusting the seating die 1/8th turn down during seating resulted in the shoulder being pushed back without increasing neck tension.
Solution:
(a) Probably only needed to anneal case mouths and re-size
(b) I did that in conjunction with polishing down my sizing button a couple thousandths using Flitz. These two actions resolved the problems.
(c) In the future, if I want a little more crimp during bullet seating I will use a Lee factory crimp die.
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