Thinking about this now it makes me wonder if you could run modern 9x23 Winchester in a mp5. I think its doubtful due to that catridges OAL length and mp5 mag dims, as well as the mp5 which in many cases is challenged in feeding 9x19 non-round nose 9x19 ammo. However if you could get past those issues, you would get into 357 and high end .40 cal to to low end 10mm territory velocity wise with the benefits of 9mm like recoil and mag capacity. where is @chopstix _Kid , @ghilliebear2000 , or @hk21man when you need them.
EDIT: I did talk to ralph at RTDS today and he did say using a 9mm chamber reamer you probably could get a 9x23 to chamber and headspace pretty easily, and use the F style recoil rod and many LPs available like say and 80 degree LP if needed, its probably feasible to get something like this firing. But again ammo fitting and feeding from the mag is the limiting factor to make it work.