With 1 step GC maker the gas checks already come out slightly dome. Making a forming pin a dome shape is a 5 minute task on a lathe but making a concave pin is more complicated but I don't think it needs to be matching, as long as you got a ring of support pushing on the edge the cavity doesn't need to be exact dome shape. Also as Tommy mentioned you can size it with a push through die with a gas check head first. What caliber you're trying to make that for?
P.S. I just realized that I have a round end mills, this should be easy.
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