You are correct about the ever so slightly larger 7wsm case, however I think what skip just posted makes the most sense for necking up, 325wsm makes more sense. .284 to .338 would be very hard on a case neck. .308 to .338 is more easily achieved, but (325wsm/8mm) .323 to .338 is one pass thru the die, and I would think that would promote much better case life and ease you into wildcatting with a few less steps.
Ive thought about this very project, and that is what I would do if it were me. Unfortunately I dont currently have the funds to give it a go, or Id try one with you
Looking forward to seeing how it turns out. Good Luck
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