Now with an edited sound!
Standard Version
As it turns out, the rate of fire is capped to your FPS, which is pretty bad in Fallout 4; Myself it's rarely above 30 in outdoor areas. Each of the barrels has the vanilla rate of fire, but fire an extra projectile instead. The accuracy is slightly reduced, just like in a two-shot legendary weapon.
High Performance Version
This version is closer to the initial version, standard and tri-barrel miniguns have their boosted rates of fire, but accelerated is changed from ROF higher than the game's frame rate to standard that fires an extra projectile as in the standard version.
The rates are (As rounds per 10 seconds, like displayed in game): Standard 600, accelerated 600 with two projectiles each shot and tri-barrel 400.
Wasteful as all Hell, but somehow I don't care.
Future plans
So far I can't find a way to edit ammo consumption or get the gun to eject multiple casings. As soon as I find a way to do that, there will be a new version.