Without annealing every piece of brass, you WILL get variations in shoulder bump. There's simply differing amounts of "spring back". Now if the round chambered, the bolt could have bumped that shoulder back enough that it checks out in your chamber gauge. You want your die set so that it's averaging in the middle of the min/max headspace. Too little sizing, and you get sticking rounds. Too much sizing/bump, and you'll get separations.
Could just be a chamber on the tighter side too that's playing a role in this.
"What would a $2,000 Geissele Super Duty do that a $500 PSA door buster on Black Friday couldn't do?" - Stopsign32v
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