Another internet myth , started by people who did not understand what they read . Friends in Finland do not think this . The cases are the same , within normal military tolerance . Just look at real ammo . I have a good amount of both . About half of the Finn used rifles and ammo was captured USSR . The case is right at 53.5 , so it is rounded up or down . Some civilian club rifles had .308 ish bores and used .308 match bullets and were later used in war . USSR WWII era ammo was made with a .3098 dia bullet , not .311 as thought by some . Most militaries have slight variations in different loads of their service ammo .