Jagermeister,
The 307Win is a rimmed 308(at least simi-rimmed). If you are having something built, it is easy enough to have it chambered with a 308 reamer snd single point the rim recess for 7x57R(8x57R,7x65R)rim dia.Make cases with a 308 file/trim die and load with standard 308 dies and shell holder to fit the case.This cartridge would be great, but too much for a small martini.
Mike