The experimental MIRV is a unique Fat Man that can be found in the bomb shelter area in the National Guard depot. This weapon deals the most damage of any weapon in the entire game, as it acts like a shotgun version of the Fat Man, delivering eight mini nukes at once. The MIRV actually does the same damage as a Fat Man per mini nuke, as 12,880/8 equals 1,610. This means that the MIRV is simply a Fat Man with extra capacity and increased ammunition usage. It will not fire if you don't have at least 8 mini nukes in your inventory to load it with. If you have fewer than 8 mini nukes, the weapon will reload but never discharge the payload.
The player must complete the unmarked quest Keller Family Refuge before they can gain access to the secure section of the depot housing the weapon, though it's also possible to gain entrance to the armory by using a glitch. See the National Guard depot bugs section for more details.
The MIRV looks identical to the Fat Man, meaning only 1 mini nuke appears to be loaded on to the weapon; however, the game overlaps all eight mini nukes, which separate upon exiting the weapon.
Sometimes the MIRV (or Fat Man), is set up such that using it in V.A.T.S. will cause the player to aim directly at the enemy, without aiming upwards to compensate for the distance. This, coupled with the fact that only the 8th nuke shot out of the MIRV will go as far as the Fat Man's single nuke (and all other nukes will travel shorter distances), means that using the MIRV in V.A.T.S. will cause the player's death.
If you look directly at the ground and fire the MIRV, the 8 mini nukes' combined power is enough to launch your dead body to the height limit (roughly the height of Tenpenny Tower).
Similarly, the MIRV can potentially send the bodies of human enemies hit by multiple mini nukes flying hundreds of feet into the air, potentially rendering their bodies impossible (or very hard) to find. Keep this in mind before using the MIRV on an opponent whose body you want to loot.
It costs more to fire this weapon once than it does to repair a regular Fat Man twice. At several hundred caps per nuke and firing eight nukes per shot, this is the most expensive weapon in the game, albeit the most powerful.
Depending on your game system, its age, and the amount of excessive use of the MIRV, it may cause your system to lag and possibly crash.
Out of every weapon in Fallout 3, the MIRV is the most powerful in terms of base damage.
Considering everything mentioned above, the experimental MIRV is one of the most inefficient, unnecessary, and impractical weapons in the game, if not the series. However, with an ammo console command (PC), this weapon can easily take down large groups of more powerful enemies (e.g. super mutants, Enclave, and deathclaws).
MIRV is a military term for Multiple Independent Reentry Vehicle (or Multiple Independently Targetable Reentry Vehicle). It is a collection of nuclear weapons carried on a single intercontinental ballistic missile (ICBM). Using a MIRV warhead, a single launched missile can strike several targets, or fewer targets redundantly. The MIRV system was developed throughout the middle of the 20th Century, and one of the main ideas behind the weapon system was to take out an enemy country with a single missile attack before the enemy nation could be able to respond.
xbox360 Sometimes a graphics error can occur when detonation is very close. It can cause large, black rectangles to extend from the weapon out to as far as you can see. A console restart will fix this. [verified]
xbox360pc Sometimes when lifting the item, it will twirl around in the air, making it very hard to decorate your Megaton house or Tenpenny suite with it. [verified]
pcIf you equip the weapon and enter V.A.T.S. while having less than 8 Mini nukes, V.A.T.S. will not show you the "not enough ammo" box, but if you actually try to use it the weapon will not shoot and you will get stuck in the V.A.T.S. sequence, reloading a save will fix the problem. [verified]