>>96260290
The drum was meant for when units are on the move, so the machine gunner has the weapon ready to fire the moment they make contact and gives the loader time to get the bigger belt ready. You're also running through all that ammo if you're brrrrt it all away. At least with the MG3, the idea is to fire very short bursts:
https://youtu.be/ENipa4fYOSc?t=29
It's the whole reason for this meme:
https://www.youtube.com/watch?v=7kZ95uYIlgc
He's probably been trained to fire quick bursts with the MG3, something that doesn't not work as well on the M2.
>Meanwhile, the heavy bolter backpack feed is more like GAU8 on A10 warthog with 1,350 rounds of ammo.
Militaries have experimented on backpack mounted ammo boxes with belts to the gun and they never end up working. Meanwhile, belt boxes of shapes and sizes been used for over a century by now. So you can cry about how they're shit all you want, they're still a thing actively being used, rather than some designers wet dream that never goes anywhere.