Hot enough, in fact, that it will eventually start spontaneously igniting (or "cooking off") the bullets without waiting for you to pull the trigger. Congratulations, you now are holding a possessed machine gun that is randomly spraying bullets on its own, which is enough to make even the most grizzled member of the Expendables cast shit his pants:
Now imagine the above happening when you're, say, running, or have just climbed into a vehicle, or any other situation where you totally don't want to be shooting a gun at that particular moment (life is actually full of such situations, if you think about it). And that's the best-case scenario -- when ammunition cooks off, things can get uncomfortably explodey for the man behind the gun:
Yeah, notice how that happens right when your face is pressed up against it. (Wait, is this why the aforementioned Expendables guys all look like they've had so much plastic surgery? From the face burns?) That's why, in real life, these fragile, temperamental weapons have to be used in careful coordination -- not only is a guy with a machine gun not a one-man army, he's literally not allowed to be -- the Marine Corps manual, as one of its first rules of machine gun deployment, states: "No machine gun should be placed in isolation." There are many reasons for this, but the unspoken second half of that sentence could well be: "unless you want to catch your own face on fire."
As Matt, our former Army Ranger, says, "There's a science to using machine guns. Let's say you've got three big guns in a weapon squad. They aren't just picking targets and shooting -- and they sure as hell aren't firing at random. They fire as part of a unit. Gun 1 fires a short burst, then Gun 2 fires, then Gun 3 fires, and then back to Gun 1. This is called talking."
"Hey, why don't you guys come out, so you can hear us a little bit clearer."