The Britishicon .303 and the Jap 7.7 Rimmed are the same round for all practical purposes . Headstamps would be different . The Japs MAY have purchased .303 from the Brits early on ( 1938 / 39 ish ) to get started and I'm certain they would use any captured supplies they could . I was just saying that could be a reason a British round might have gotten on a US / Jap battlefield .
Don't know enough about the two countries MGs to know if the firing pin shapes differed or not .
The Japs also adopted a copy of the Germanicon MG-15 in 8MM Mauser for airplane use mainly in China , so 8MM could be found on a battlefield that the Germans never was on , too.
Chris