how about a 7MM SHOOTING TIMES EASTERNER, it was invented by Layne Simpson of shooting times , its a .307 win. case necked to 7mm and slightly improved , .307 win. brass is supposed to be to be able to be made from .444 Marlin bras. Unfortunately , flat nose 7mm bullets may be hard to find , there were 120 gr. and 139 gr. made by hornady , I suppose there are others .( for the 7/30 waters)
hawk bullets says they will make custom bullets but there's an extra charge
the 120 gr. bullet reportedly moving out at 2900 fps
you can google it if you want , there are plenty of postings
edit : maybe it wasn't invented by Lane Simpson , but the cartridge is credited to shooting times