Well, there are various different issues here. One of them is the length of time for which the firepower is sustained; any more than short, widely-spaced bursts and the key constraint isn't rate of fire or magazine/belt capacity, it's barrel heating.

There are three different levels of performance in dealing with barrel heating:
air-cooled fixed barrels (Chauchat, BAR, Browning M1919), air-cooled quick-change barrels (Bren, MG 34/42), and water-cooled barrels (Maxim, Vickers, Browning M1917).

With quick-change barrels, the constraint is the number of barrels available to keep switching. With water-cooled barrels, there is no constraint provided that you don't run out of water and ammunition!

__________________
Tony Williams: Military gun and ammunition website