New Army "Caliber Configuration Study"

[stanc]

I think that the proposed .276 Penderson and the .280 British (the 7x43mm version) could have been GPC's. They were intended for individual weapon and LMG use. I don't know if they were to be used in the MMG role, however.

IIRC, a .30-cal M1917 HMG was converted to .276 Pedersen. (The US did not have MMGs in WWII, only air-cooled LMGs and water-cooled HMGs.) However, .276 Pedersen may not have been a good GPC, as it reportedly was inferior to .30-06 in armor penetration.

Ironically, the Japanese had the 6.5mm Arisaka and the Italians had the 6.5 Carcano, but both initially used round-tipped bullets which had poor terminal ballistics. They both decided to move up to to a larger caliber. I do not know if the Japanese reevaluated the 6.5 Arisaka after switching to the pointed bullets which would yaw after impact.

The Japanese adopted spitzer bullets for 6.5 Arisaka prior to the First World War, so they would've had plenty of time and opportunity to evaluate it in combat.