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As above bro, you have a fundamental issue here....all good holding good groups out that far...I'm surprised.
I wouldn't be happy to continue shooting those loads myself

Its odd...Could you borrow someone's dies to see if it remedies or repeats?

as above, it doesn't look like your problem is related to seating stems

I never liked the idea of compressed loads and like ball powder partly for this reason

As above...looks like too much force to seat them. I get a faint ring on mine when seating.
I don't have a micrometer to measure so am relying on the dimensions of dies and annealing after 3 or 4...