the bigger the gun, the bigger the spinup/spindown. also on how powerfull the drive system is. miniguns in games that spin up before feeding ammo are bs. such a system in real life would jam. you get
more reliability mechanically linking the feed system to the barrels. works like a bicycle chain, think of every link as carrying a bullet, and when the chain passes betweeen the teeth on the sproket, that bullet may then be locked loaded and ejected and the casing exit on the very same link. thats pretty much how gatling guns work.
russian gatling guns are a little wierd. they use the gas from the bullet going off as sort of a truster to keep the barrels spinning. its initiated with a pyro charge (limiting the number of times they can fire it). breaks engage when the trigger is released. but the guns get insane rates of fire and almost no spinup delay, in excess of 10k rpm.
i think the rounds are linked and spin directly off a feed drum with no complex conveyor systems, which is why it gets such awesome rates of fire, less stuff to work.
as for the gun in predator, its not possible to use it in that way. its designed to be operated as a manual turret on vehicles or as an implacement (tripod burried in sandbags so as to take the recoil). it was a real minigun but the only way they managed to fire it from the waist is by shooting blanks, and limiting its rpm. nukemod has gatling guns ranging from minigun size to a 105mm gatling-howitzer