The M42 Fat Man was developed at Fort Strong near Boston, Massachusetts. Development began in February 2076, with the goal being the development of a man-portable tactical nuclear launcher easy enough for use by troops on the ground, to destroy reinforced and entrenched positions. Problems manifested instantly, as warheads were simply too heavy to clear the blast radius fast enough to provide safety for the operator. Even stripping them to the minimal possible size failed to produce an acceptable effect. Instead, the development team decided to increase the power of the launcher unit, even considering using railgun technology to accelerate the warhead. The notion was largely academic, as it would require for the overburdened soldier to carry an additional power pack the size of a suitcase and turn the Fat Man into a crew-served weapon. Concurrent development ran on the MIRV version of the launcher. Live-fire testing claimed lives, in fact, with two soldiers killed during a test. No remains were recovered and the commanding officer in charge of the project, General Brock, covered up the incident, ordering his subordinates to send sand packed in an urn to their relatives.
The project hit a breakthrough in December 2076, when the project lead discovered a solution for the launcher problems. Six hours, fourteen cups, and one concerned night shift lab crew later, the future M42 launcher was conceptualized: Based on the spigot mortar mechanism, the launcher would only have to accommodate a firing through and system, relying on mechanical launching, rather than chemical or electrical means. Much like the 130 year old PIAT launcher, the Fat Man warhead would be propelled by a sub-charge embedded inside the warhead, "catapulting" the warhead through the air to the target - giving plenty of range with the added bonus of not producing a smoke trail that could reveal the firing team to the enemy.
By September 2077, after one hundred test firings the weapon were complete and ready to be shipped to units overseas. A large number of units was manufactured and shipped together with the appropriate warheads to select locations throughout the United States.
The standard M42 "Fat Man" Launcher is a man portable nuclear payload, anti-materiel weapon platform. It can contain a single mini nuke only.
The mini nuke projectile is very heavy, and if simply fired straight ahead, it will travel only a short distance before falling to the ground and detonating (causing you to be caught well within the very damaging (and often fatal) mini-nuclear blast). For optimal range, the Fat Man should be fired in V.A.T.S. mode at high skill levels (which automatically compensates for the projectile's downward trajectory), or at an upward angle to catapult the mini nuke further so that it impacts at a safer distance.
The Fat Man has an estimated range of about 150+ yards. The blast zone will be mildly irradiated for a short time after detonation, giving out up to 5 rad/s.
The Fat Man's condition can wear down surprisingly fast with frequent use (though its firepower is still devastating at any state of repair).
Experimental MIRV, an incredibly destructive weapon, the MIRV fires eight mini nukes in a single shot.
White House - in the ruins, near a skeleton. Next to the Fat Man are three mini nukes. The White House can be accessed from a Utility manhole at White House Plaza, south of the building on Pennsylvania Ave.
The names of Fat Man and Little Boy are references to the code names of the atomic bombs dropped by the United States on Japan during World War II: Little Boy was dropped on Hiroshima while Fat Man was dropped on Nagasaki. For this reason, the weapon's name has been renamed as the Nuka Launcher in the Japanese translation of both Fallout 3 and Fallout: New Vegas due to its relation to Japanese history. However, the Little Boy modification still retains its name.
The bell heard after reloading is actually the lunch bell at Bethesda.
The designation M42 is shared with a couple of other pieces of military equipment, such as the M42 Duster SPAA gun and the M42 submachine gun.
The M42 Fat Man is similar to the real-life M-28/M-29 Davy Crockett miniature nuclear launch device.