Think of machine guns and rounds, like a fat person thinks of cake. It's not the quality, but the quantity. A fat person(The horde of charging huns you want dead) isn't going to be satisfied(dead). With just one slice of the best cake(bullet) in the whole wide world. They might be a little satisfied with a little bit of cake(bullets). But you want them to be really satisfied(all dead). So you fling as many fucking pieces of cake(bullets) in their direction, as fast, and as violently and with as little stoppage as possible.
I don't know the mag change procedure for that type of gun, but if I am changing mags every 20 rounds with a machine gun. I am going to be crazy pissed off, as well I won't be effective as suppressing fire stops even briefly every time I do. Sometimes you just have the primal need to put 400 rounds through someone's house. Because fuck him, fuck his furniture and fuck everything around him. That gets hard to do when every 5 seconds you have to change a mag.