How about a 10 mm necked down to .308 cal? It would require the use of short ogive bullets. Could be along the lines of the 9x25 Dillon. The 7.5 FK BRNO is longer than the 10mm and runs at higher pressure, but it's a really a 30.

7.65×21mm Parabellum

3mm shorter

parent is

7.65×25mm Borchardt

Yes, but I want BIGGER. I play with the 30 Mauser/7.62x25 Tok a bit. Unrelated tangent: I've read Sig originally headspaced the 357 Sig on the shoulder then changed to the case mouth. Why?