6mm rounds seemed to be common in pre-War II era: Swedish Mauser, Arisaka, Carcano (aka "JFK assassination rifle"). Interestingly enough, the Fedorov Avtomat --

6mm rounds seemed to be common in pre-War II era: Swedish Mauser, Arisaka, Carcano (aka "JFK assassination rifle"). Interestingly enough, the Fedorov Avtomat -- the first select fire rifle -- used the 6.5 mm Arisaka round (from the captured Japanese stockpiles after war of 1905).