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Are you using Brass from a single mfg/single batch of brass? If not there could be variations in seating pressures/neck tension between cases?

I once had similar problems with Remington Saum...

Once an immunologist by any chance? Is the sizing die a bushing die or does it have an expander mandrel? You didn't change bushings or remove the mandrel (be pretty hard to seat into a case resized...