Experimental MIRV

The experimental MIRV is a unique Fat Man that can be found in the bomb shelter area in the National Guard Depot on a table near the remains of the skeletons of the Keller Family, who play a central role during the player's acquisition of this weapon. This weapon deals the most damage in the entire game.

Obtaining
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.

It's also possible to gain entrance to the armory by using a glitch, see National Guard Depot for details. Just go into Third-Person View, crouch, and move your analog stick [or mouse] and look for a different activation line saying "Activate Electrical Switch". You can now get the MIRV without having to find the Keller Family Transcripts. The Electrical Switch will be slightly lower and to the left of the Terminal. A Glowing One will be found at the back of the cave/secret room.

Background
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.

For a good example of a MIRV, and possibly the real-world weapon this gun was based on, see the LGM-118A Peacekeeper ICBM system developed by the United States during the Cold War. It was capable of firing one missile which would 'birth' up to 10 nuclear warheads over a wide target area.

Bugs

 * Sometimes NPCs in towns attacked with the MIRV remain hostile after reloading the game. The best way to prevent this is to save outside the town, enter it, blow it up, and load the save. If it happens, rebooting your console may help, as it will clear all data in your console's RAM.
 * 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.
 * Rarely, when you bring out the MIRV, there will be a Mini-Nuke stuck on the end of the weapon. This is purely a graphical anomaly, more likely to occur when using the console command 'tgm' (enabling infinite ammo, god mode, no reload), as this weapon was not intended for rapid-fire.
 * You can enter V.A.T.S. if you have fewer than 8 rounds. It will 'click' 8 times over the course of 20 seconds then exit V.A.T.S. with no action point loss. Since you only take 10% of normal damage whilst in V.A.T.S., this can allow for strategic use in difficult situations (ie if you're standing next to a car that's about to explode).
 * Sometimes when carrying the Experimental MIRV attempting to enter Tenpenny Tower can cause the game to freeze.
 * Sometimes if you have 8/8 ammo and fire both clips, the MIRV can go invisible in 3rd Person, but in 1st Person, it's visible.

Videos
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Eksperymentalny MIRV