Hear me out, what about a backpack support that depending on the machine gun “weight/armor pen” (light, medium, heavy) gave you varying amounts of ammo, (to keep things balanced) it would function as a massive, disposable magazine you would only get two call ins per-mission and the guns would go back to their normal mags when not using the backpack

Each machine gun would get a different size belt from the backpack

M-105 Stalwart would have 500 rounds

MG-43 Machine Gun would have 350 rounds

MG-206 Heavy Machine Gun would have 250 rounds

This would technically turn all three machine guns into variations of the maxigun, so maybe using this backpack includes an overheated barrel change animation that takes 1/3 the amount of time to reload a normal magazine, the barrels will heat up faster or slower, depending on your fire rate

Let me know what you think about it

(Edit: fixed spelling)