These are much heavier rounds. BAR gunners carried something like three hundred rounds tops and had an assistant gunner/riflemen humping the rest. For reference, the web gear for a BAR gunner was 12x20round magazines of .30-06, which is a significantly larger (heavier) round than the 5.56.

This is much of the reason why the 5.56x45mm round exists in the first place -- more shots, less weight.