Sincethe headstamp is from Finland I would guess it is a 7.62X54 Russian. The Russian round was used by the Finns who captured and rebuilt Russian military bolt action rifles. I have two such rifles in my collection that were remanufactured by Saco of Finland. The ballistics or the 7.62 Russian cartridge is very similar to our modern day 7.62 NATO cartridge that is still used by our military primarily in the M60 machine gun. The Russian cartridge however is a rimmed case and doesn't feed well in an automatic weapon. Monty