I been digging through all my old boxes (I moved 18 years ago; not everything is unpacked yet) and found a conversion I did many years ago...A C96 Mauser re-barreled for a .256" wildcat cartridge...
My usual .223"/5.56mm cartridge cases cut down so that OAL will be the same as a 30 Mauser (which this once was). .256 Winchester Magnum chambering reamer in a .256" barrel blank turned down & threaded into a 30 Mauser receiver. It was much longer originally (I posted a pic of it back then) but I shortened it for convenience. Also to retain what little hearing I have left - This cartridge made a really loud crack...
RCBS .256 Winchester Magnum die set cut down to '30 Mauser' size...I fired off most of my reloads, had a couple left over...