I was just recently watching an older gun jesus video (see below) where he was talking about the Vickers gun. They too used to be fired on fixed bearings and elevation at great distance as an area denial weapon before light weight mortars replaced them. When taken out of British service in the late 1960's they still had 5 million rounds left for the guns, so what to do? Easy, set up one gun at the firing range, run it 24/7 with crews that switched every 30 minutes, and blow through all 5 million rounds! It took around 9 days and when stripped down at the end, it was still within manufacturers tolerances. I suppose that's the difference between a heavy MG and GPMG.

As for the L1A1, I was reading a book about the Australian SAS in Vietnam where a few enterprising souls cut off their SLR's flash hider and used a 30 round magazine first up, and afterwards switching back to 20 round mags. Their view was that the sheer noise, flame and volume of fire when first in contact with a larger force would confuse the hell out of the enemy and gave them a chance to scoot outa there. One wag mentioned the large flame out the front was enough to inflict the wound, and then cauterise it too...

Vickers Heavy Machine Gun - YouTube