"Heavy ball" is essentially machinegun ammo. The heavier bullet has better ballistics for the MG useage. Some nations, like Finland, designed their rifle barrels to work best with the heavy ball ammo, thus making logistics easier - only one type ammo to stock! My Finn M-39 really prefers the heavy stuff.
My old M1891 and hex-receivered 91/30 prefer the light ball.
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