The 6.5×50 Japanese cartridge has a semi-rim with a weirdball diameter — just right to accept the 7.62×39 Russian cartridge. A number of Model 38s have been converted to 7.62×39.

There’s one other cartridge that will fit an unmodified Model 38 bolt recess — the .50 Beowulf. And I just happen to have a too-butchered-to-restore Model 38, a brand new .50 barrel, and a .50 Beowulf chambering reamer waiting for me to finish getting my new lathe set up.