Experimental MIRV

The Experimental MIRV is a Fat Man Launcher that can be found in the bomb shelter area in the National Guard Depot next to 5 Mini Nukes on a table near a Glowing One, near the remains of the skeletons of the Keller Family. When this weapon is fired, a random shotgun spread seems to be the path they take, usually 6 or 7 near each-other and one off in the distance before impact, if aimed correctly, this weapon can kill anything in one hit, even the Super Mutant Behemoths. You must have obtained all five of the Keller Family Transcript's before you can gain entrance to this part of the room. It will only fire eight Mini Nukes at once, no more no less.

Be sure to never use this weapon at close range or near NPC's wished to be kept alive.

Bugs
If you have fewer than 8 rounds, you can still enter VATS to attack. However, it will just 'click' 8 times over the course of 20 seconds, then exit VATS with no action point loss.

Cheat
For the PC version, pressing Console and entering "TGM" will toggle god mode, which also grants unlimited ammo (no need to reload). You can get 5-6 shots off before the first ones start detonating. Use your imagination. Also note, if you do this, your framerate will suffer more damage than the random bit of wasteland you're blowing to pieces.