I hate discussing semantics anyway, it’s just distracting from what actually needs to be discussed. The fact is we have a video of a real human being firing a real M134 minigun, unmounted. You can do it, even if you need to deal with the weight and bulk of the ammo, even if you need to be at peak human condition, even if it will tire you out and break your back after more than a few bursts. No matter how stupid and impractical it is, it is an obvious fact that yes you can fire a minigun unmounted. It is no longer a question of whether or not you can do it at all, but what price the player will have to pay to do it.
For the average, unenhanced survivor, yes firing a minigun will be infeasible. They won’t be able to carry the ammo with them, they’ll have to haul it around on the ground. The M134 itself will be too heavy to hold steady, and if they manage to lift it it will immediately be knocked out of their hands as soon as they pull the trigger.
Moving up a step in usability, a very strong (let’s say 12 strength minimum), unenhanced survivor, our Mr. Gronk, will be able to carry the minigun and pull the trigger without losing control. However, they will still have very high spread, slightly reduced by higher strength, and it’s possible that firing will strain/apply pain, even use up some stamina. Strength-enhancing mutations and cybernetic enhancements would make the recoil and weight more manageable, allowing for greater sustained usage, but they used would still have to take a break within minutes. Maybe a 10 ft tall, Huge, Insanely Strong Ursine mutant could go even further beyond.
Regarding the ammo, it has been established that the ammo is very heavy, potentially outweighing the gun itself. In most cases it would be most practical for ammo to be fed from a source next to the player, rather than being carried, and someone would have to add support for this function, for belt-fed guns to feed from the environment. It might be possible to carry the ammo on your person (soldiers and firefighters are expected to carry similarly heavy loads), but it would of course be physically demanding, requiring an exceptionally strong character, slowing them down and forcing them to take breaks and/or sustain pain from the effort (things that already happen to overburdened characters in-game).