I think you can still find 22-250 Lapua brass . Anneal & step up .243 then 257 U will be fine. https://www.powdervalleyinc.com/product/lapua-22-250-rem-100/
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There is a difference in the neck shoulder junction. Im now running a 7mm expander thru 6xc and necking back down for a false shoulder before fire forming.I would run some 6xc brass through 250-3000 dies and measure it.