wnchester sez:
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Next, fired cases have expanded and shrunk back a thou or two from full chamber length. Meaning there's absolutely no need to size shoulders further back from the fired length.
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Except when the bolt face is out of square with the chamber axis and the case gets oriented 180 degrees out form where it was previously fired. The high point on the case head will be aligned with the high point on the bolt face and the bolt will bind a bit when its closed.
If you don't set fired case shoulders back a bit using full length sizing dies, that shoulder will get moved forward a few to several thousandths by the die sizing down the case body before the die's shoulder starts pushing the case shoulder back. So one needs to set their full length sizing die such that no fired case ends up with too long of a case headspace after full length sizing.
Benchresters learned this a decade or two ago when they finally realized proper full length sizing produced better accuracy. Their smallest group sizes didn't change at all, but their larger ones got a lot smaller so aggregate groups and scores were better.