Fat Man (Fallout 3)

The Fat Man is a handheld tactical nuclear catapult, similar to the British PIAT from world war 2, found in Fallout 3.

Appearances
The Fat Man appears only in Fallout 3, and can be found in the Rivet City market area and also inside Germantown Police HQ. Because of its nuclear payload, it is by far the strongest V.A.T.S. enabled weapon in the game, perfect for taking out multiple enemies or heavily armored units - and in normal use, you'll be encountering both. The Fat Man has an estimated range of 150+ yards and will fall if shot in the air; be aware that the blast zone will be irradiated. You can find this gun right outside of GNR, as well, during the "Following in His Footsteps" quest by picking it up from a dead Brotherhood soldier. Using this method, one can acquire the Fat Man fairly early in the game. Note that this Brotherhood soldier must be killed by the Super Mutant Behemoth you encounter during this quest.

A Fat Man can also be found in Evergreen Mills, on the ground floor armory of Germantown Police HQ, or at the Talon Company fort. It may also be dropped by Talon Mercenaries in the capitol building during or after the fight with the Super Mutant Behemoth.

In the Armory at National Guard Depot, behind a door that requires a passcode, you can find the Experimental MIRV version of Fat Man which can fire up to 8 mini nukes at once. This is no easy task, as all 5 Keller Family Transcripts must first be found.

One can be found at the end of the Old Olney Sewers after battle a number of Deathclaws although this may have been a random find as I haven't anyone mention finding one in this location.

One can also be found in Fort Constantine Alongside the T-51b Power Armor and two Mini-nukes. You will need all 3 of the special keys from the You Gotta Shoot 'Em in the Head Quest to access the room.