Because you are thinking Martini, that will pretty much limit you to a rimmed case. Rogervich is about on. Something like a K Hornet or 218 Bee necked down would be the most practical. Trying to use something like a 38 Special case will cause fits necking down. Too radical; like making 256 Winchesters from 357. Would probably have to neck to 30 then 25 then 22 then 20, or something on that order. Even with the extra case capacity I doubt you would really need it or want it. The small cases would be easier all around.