"I never questioned the 12,000 round claim. That's what we'd normally draw for a day at the range per two guns, we'd expend them all, normally well before we were scheduled to end. That includes being down for periods of instruction, breaks, chow, swapping out gunners, more instruction, etc.
The MG42 while having a theoretical ability to fire 1100-1200 rounds per minute has a practical rate of fire in the 250-300 rounds per minute range, not much different from it's contemporaries or modern day counterparts. To make a long story short, the "mg42's bullet output" isn't really a factor because even using most of the 42's contemporaries 12,000 rounds isn't really that hard to do. 300rpm x 4 minutes=1200 rds x 10=12,000 rounds; so 4 minutes times 10=40 minutes of hard sustained fighting. If he'd had a water-cooled a la a M-1917, Vickers or MG 08 you could get a higher round count because you minimize the over-heating factor. "