The Heavy Gunner perk increases the damage dealt by heavy weapons such as miniguns, missile launchers, Fat Man, etc.
Also:
The "Explosive" legendary modifier adds an immense amount of DPS to the minigun, particularly with the last ranks of Heavy Gunner and Demolition Expert
(76-80 damage per shot displayed, compared to 16-20 without any legendary modifiers and depending on barrel mod).
The splash damage this combination creates is incredibly brutal and capable of wiping out whole groups of clustered enemies in a single burst, but it will hit anything in the area including settlers, town citizens and other allies.
Another huge bonus is the fact that the minigun's considerable spread becomes largely irrelevant - as long as the gun is aimed at a target, it will hit it even if the actual projectiles don't connect.
Not even cover can keep targets safe from the devastation this gun unleashes.
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The minigun is tied with the Gatling laser as the ranged weapon that makes best use of the "Furious" modifier, mainly due to their insane rate of fire.
Keeping such a gun aimed at a single target will quickly increase the damage of each shot to absurd levels,
making it very useful for taking on giant creatures like Super Mutant Behemoths or Mirelurk Queens as well as other hostiles that boast a lot of hit points.
So, there is nothing wrong with the minigun.