RE: Fallout 3
The Experimental MIRV (Strongest Weapon)
The Experimental MIRV Fat Man Launcher can be found in the bomb shelter area in the National Guard Depot Armory 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 1 hit, even the Super Mutant Behemoth. You must have obtained all five of the Keller Family Transcript's before you can gain entrance to this part of the room. It fires 8 Mini-Nukes at once and if you have less than that it will just make an error sound telling you that you do not have enough ammo.
In appearance, it looks like a rustier version of the Fat Man, and when loaded the Mini Nuke projectile(s) appear to be in a bundle.
As i said the Keller tapes to be to found before you can gain entrance to the room its located the tapes can be found:
Fast Travel to VAPL-58 Power Station and head North following the power lines, the 3rd power line down will be surrounded with boards on 3 sides, head inside and on the North wall will be a bookcase with the 1st Keller tape.
Fast Travel to Big Town and head North up to the Hallowed Moors Cemetary, going in the front entrance, the 2nd Keller tape will be on the left on top of the podium.
Fast Travel to Grisly Diner and head inside, then goto your right and out the back, the 3rd Keller tape is located on a desk at the back.
Fast Travel to Rockbreaker's Last Gas and turn to the west, you will see an abandoned shack at the top of a cliff, this shack is the location of the 4th Keller tape. Make your way around the cliff from the left side of the mountain. Enter the shack and the tape will be on the workbench on the left. (Note: After the The Waters of Life quest is completed, an Enclave soldier with a sentry bot will be vertibird dropped across from the shack)
Fast Travel to Anchorage Memorial and head to the North, quickly you will get to the edge, across the chasm you will see a broken down truck and a tent. Make your way across the river to the tent, inside the tent on the table is the location of the 5th Keller tape. Note, there is a Super Mutant or Super Mutant Master and 2 Centaur there. If you are a first time visitor, there is also a Captive inside.
Also inside of the shack containing the 4th tape, inside the locker is the Victory Rifle (a stronger version of the normal rifle, this can be reparied with normal rifle's and also uses the same ammo)