Someone on Reddit actually did the math on this 5 years ago.
The scene in Predator where someone holds down the trigger and fires continuously for 45 seconds is actually vaguely possible.
The gun weighs 39 kg or 85 lbs, so it's possible for a soldier to carry it, especially the soldiers in Predator who are shown to be much, much stronger than the average soldier. The ammo used in 45 seconds would weigh 25 kg, so it's still vaguely possible that the soldier could carry it along with the weapon. The volume of ammo wouldn't be that much of an issue. It's under 1000 rounds for 45 seconds of fire, which would fit in a backpack-sized box.
Another big issue is the recoil. On earth, someone experiences 9.8 newtons per kg of mass. We don't know how much Bill Duke weighs, because he's just an actor, but since Jesse Ventura used to be a "wrestler", we know his billed weight of 111 kg in his wrestling days. So, without any gear on he'd be experiencing 1088 N of force from gravity. Add the weight of the minigun and its ammo that's 175 kg, or 1715N.
The recoil generated by the gun is based purely on the rounds it fires. An online calculator puts the recoil impulse at 13.4 Ns of impulse per bullet. From that reddit post, they figured out the minigun in that scene had been slightly slowed down from the normal 2000 rounds per minute (33.3 rounds per second) to only about 20 rounds per second. But, even then, that means the gun would be generating 268N of force. So, just to avoid being moved, someone would have to lean atan(268/1715) = 8.8 or about 10 degrees forward. That may not sound like a lot, but when you're already carrying the equivalent of another person from the weight of the minigun and its ammo, that's a lot of extra force to deal with.
In many games, you can move normally while carrying a minigun, but as soon as you start firing it you slow to a crawl. They actually got that part right. It would be hard to move freely while this thing is shoving you back with such force.