No balistics given but extrapolating somewhere between .360#2 and .375 Flanged 2 1/2"

Actually could be a nice medium. Chamber pressure might be a bit high to get the velocity in a x57 case, so eventually would end up about 375 2 1/2 I would think.
Easier way would be to neck up 9 x 72 R I would imagine. Extra case capacity could maybe get the velocity without the pressure.

Regards