"A multi-barrel machine gun with an extremely high rate of fire. Mechanoid-built."
The minigun is a Mechanoid weapon which can be found equipped and dropped by Centipedes, or made at a Machining table. Although the Minigun has a very high rate of fire and a moderate warmup and cooldown time, it is generally impractical to use outside of point-blank to very close range combat (even with higher qualities) due to the insanely quick accuracy dropoff. It may be a better idea to sell it to suppliers who will pay quite some money for one, or smelt it for a little steel.
Miniguns are excellent crowd control weapons. Their miss radius and extremely high DPS are valuable assets when it comes to crowd control, especially in later game when there are many more enemies in raids. If you manage to get multiple miniguns to your name, you could possibly have a great deal of firepower on your side, but also a high risk of collateral damage if within your base, dependent on setup.
The minigun is even more powerful if equipped on trigger-happy colonists, as they have a 50% reduced warm-up time, allowing them to fire much faster; the accuracy penalty is largely irrelevant for this weapon.
A single centipede armed with a minigun is almost trivial to deal with if engaged at even medium range and behind a wall or sandbags, or simply spreading out sufficiently; stack up multiple miniguns with Inferno Cannons and Heavy Charge Blasters and things can quickly get out of hand without killzoning, (ideally at least 6) mortars or just outright raw firepower.
As of Alpha 16 (December 20th, 2016),, heavy mercenary raiders may be equipped with a minigun.