Editing Nuclear weapon design (section)

===Gun-type assembly===
[[File:Gun-type fission weapon en-labels thin lines.svg|thumb|350px|Diagram of a gun-type fission weapon]]
{{Main|Gun-type fission weapon}}
[[Little Boy]], the Hiroshima bomb, used {{convert|64|kg|lb|abbr=on}} of uranium with an average enrichment of around 80%, or {{convert|51|kg|lb|abbr=on}} of uranium-235, just about the bare-metal critical mass {{xref|(see [[Little Boy#Assembly details|Little Boy]] article for a detailed drawing)}}. When assembled inside its tamper/reflector of [[tungsten carbide]], the {{convert|64|kg|lb|abbr=on}} was more than twice critical mass. Before the detonation, the uranium-235 was formed into two sub-critical pieces, one of which was later fired down a gun barrel to join the other, starting the nuclear explosion. Analysis shows that less than 2% of the uranium mass underwent fission;<ref>Glasstone and Dolan, ''Effects'', pp. 12–13. When 454 g (one pound) of <sup>235</sup>U undergoes complete fission, the yield is 8 kilotons. The 13 to 16-kiloton yield of the Little Boy bomb was therefore produced by the fission of no more than {{convert|2|lb|g}} of <sup>235</sup>U, out of the {{convert|141|lb|g}} in the pit. Thus, the remaining {{convert|139|lb|kg}}, 98.5% of the total, contributed nothing to the energy yield.</ref> the remainder, representing most of the entire wartime output of the [[Y-12 National Security Complex|giant Y-12 factories]] at Oak Ridge, scattered uselessly.<ref>Compere, A.L., and Griffith, W.L. 1991. "The U.S. Calutron Program for Uranium Enrichment: History,. Technology, Operations, and Production. Report", ORNL-5928, as cited in John Coster-Mullen, "Atom Bombs: The Top Secret Inside Story of Little Boy and Fat Man", 2003, footnote 28, p. 18. The total wartime output of Oralloy produced at Oak Ridge by July 28, 1945, was {{convert|165|lb|kg}}. Of this amount, 84% was scattered over Hiroshima (see previous footnote).</ref>

The inefficiency was caused by the speed with which the uncompressed fissioning uranium expanded and became sub-critical by virtue of decreased density. Despite its inefficiency, this design, because of its shape, was adapted for use in small-diameter, cylindrical artillery shells (a [[gun-type fission weapon#US nuclear artillery|gun-type warhead]] fired from the barrel of a much larger gun).{{Citation needed|date=October 2023}} Such warheads were deployed by the United States until 1992, accounting for a significant fraction of the <sup>235</sup>U in the arsenal{{Citation needed|date=June 2021}}, and were some of the first weapons dismantled to comply with treaties limiting warhead numbers.{{Citation needed|date=June 2021|reason=Very doubtful given the only treaty dealing with tactical weapons was the intermediate-ranged nuclear forces treaty}} The rationale for this decision was undoubtedly a combination of the lower yield and grave safety issues associated with the gun-type design.{{Citation needed|date=June 2021|reason=W33s were stored disassembled}}