The MIRV launcher changes the Fat Man to launch six mini nukes simultaneously instead of just one, or twice as many if used on the Big Boy. It still only causes one mini nuke to be used per shot, but can significantly increase the damage output of the weapon. The mini nukes are initially clustered together after firing, but separate midair after about 1 second, similar to a cluster bomb. Upon separating, all of the mini nukes will fall down, rather than continue flying, effectively turning the Fat Man from a nuclear catapult into more of a nuclear artillery strike. Its range is also far lower, due to the nukes losing almost all horizontal momentum upon separating.
Nuclear material (6)
Gun Nut (4)
MIRV launcher (1)
This mod cannot be found anywhere in the game, but it can be crafted at any weapons workbench if the material and skill requirements are met.