Where are you getting that stat of 100K rounds per kill?
Back in the Civil War, 100K rounds was one heck of a lot of ammo, and they had some hugely expensive engagements in a single day - I have to believe that the RPK in the Civil War was much lower than 100K. The engagement distances were much lower, too.
I can believe it from WWI onwards, when we had belt-fed weapons - I can believe that easily. But pre-belt? I dunno.
It’s very possible that I misremember the start date of that statistic.