Science
GUN-TYPE DESIGN
Science > Bomb Design and Components
The gun-type bomb design employed two pieces of fissile material: a target and a bullet. When the bomb was detonated, a gun fired the bullet of fissile material into the target, assembling a critical mass and triggering fission. The idea was very straightforward, though additional features improved the bomb's efficiency. The target would be nestled within a tamper, a device that reflects escaping neutrons back into the fissile material and reduces the necessary size of the apparatus. Due to the simplicity of this design, it was never tested prior to wartime use. The Trinity test used an implosion design, which was considered more complex by physicists at Los Alamos, and so the success of this more complicated design suggested to the physicists that the simpler gun-type design would work. Furthermore, enriched uranium was still scarce, and so the Manhattan project physicists did not want to conduct what they believed to be an unnecessary test. The atomic bomb dropped on Hiroshima on August 6, 1945, nicknamed "Little Boy," used a gun-type design. 
The basic idea behind the gun-type design was simple, but the unique specifications of the bomb project required Los Alamos physicists to study complex ballistics problems to effectively assemble a working bomb. Much of this ballistics and ordnance work was led by William Parsons. An important consideration was that the time required to assemble the fissile material in the gun was long compared with the time needed for fission to occur. If fission begins before the core is fully assembled, the bomb will fizzle. This was not a significant problem for the uranium bomb, though it did prove fatal to the plutonium gun-type bomb. In April 1944 Emilio Segre's team discovered that an isotope of plutonium, plutonium-240, released neutrons in such a way that pre-detonation was unavoidable. The gun-type design was therefore limited to using enriched uranium as fuel, while the plutonium bomb needed to use the more difficult, but faster, implosion technique.
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The text for this page is original to the Department of Energy's Office of History and Heritage Resources. For further information, see Lillian Hoddeson, et al., Critical Assembly: A Technical History of Los Alamos during the Oppenheimer Years, 1943-1945 (Cambridge: Cambridge University Press, 1993) and Richard G. Hewlett and Oscar E. Anderson, Jr., The New World, 1939-1946: Volume I, A History of the United States Atomic Energy Commission (Washington: U.S. Atomic Energy Commission, 1972). The sketch of the gun-type design is reproduced from Robert Serber's April 1943 "Los Alamos Primer," 21-22. The photograph of "Little Boy" is courtesy of the U.S. Army Corps of Engineers (via the National Archives).