The Minigun is a very fast firing gun. It is notable ESPECIALLY for it's ROF, which is at extreme levels. It's damage is comparable to it's machine gun counterparts. This weapon is good for suppression fire. You can get it with Director's Cut from Arms Dealer Larry in the Dangling Pianist bathroom for $20,900. The description for it reads: Nothing very "mini" about it.
It also has a stronger version, 'The King' if you max out the job at City Hall on the fourth island.