W48 The W48 was an American nuclear artillery shell , fired from a standard 155 mm ( 6 . 1 inch ) howitzer , e . g . the M114 or M198 . It was manufactured starting in 1963 , and all units were retired in 1992 . The W48 was 6 . 1 inches ( 155 mm ) in diameter and 33 . 3 inches long . It came in two models , Mod 0 and Mod 1 , which are reported to have weighed 118 and 128 pounds respectively . It had an explosive yield equivalent to 72 tons of TNT ( 0 . 072 kiloton ) , which is very small for a nuclear weapon . Linear implosion . The W48 was a small diameter linear implosion nuclear fission weapon . Larger , " normal " implosion nuclear weapons use less nuclear material than is required to form a critical mass at normal pressure and configurations , and use large precise explosive assemblies to collapse the nuclear material to many times normal density and attain critical mass . Linear implosion uses a mass of nuclear material which is more than one critical mass at normal pressure and a spherical configuration . The mass , known as the " pit " , is configured in a lower density non - spherical configuration prior to firing the weapon , and then small to moderate amounts of explosive collapse and slightly reshape the nuclear material into a supercritical mass which then begins chain reaction , goes critical , and then expands in a small nuclear blast . Three methods are known to compress and reshape the nuclear material : collapsing hollow spaces inside the nuclear material , using plutonium - gallium alloy which is stabilized in the low density delta phase at a density of 16 . 4 ( and which collapses to denser alpha - phase under moderate explosive compression ) , and shaping an explosive and nuclear material so that the explosive pressure changes a stretched - out , elliptical or rugby ball shape to collapse towards a spherical or more spherical end shape , which decreases the surface area of the pit . A bare critical mass of plutonium at normal density and without additional neutron reflector material is roughly 10 kilograms . To achieve a large explosive yield , a linear implosion weapon needs somewhat more material , about 13 kilograms . 13 kilograms of ( highest density ) alpha - phase plutonium at a density of 19 . 8 g / cm³ is 657 cm³ , a sphere of radius 5 . 4 cm ( diameter 10 . 8 cm / 4 . 25 inches ) . Linear implosion weapons could use tampers or reflectors , but the overall diameter of the fissile material plus tamper / reflector increases compared to the volume required for an untamped , unreflected pit . To fit weapons into small artillery shells ( 155 mm and 152 mm are known ; 105 mm has been alleged to be possible by nuclear weapon designer Ted Taylor ) , bare pits may be required . Linear implosion weapons have much lower efficiency due to low pressure , and require 2 - 3 times more nuclear material than conventional implosion weapons . They are also considerably heavier , and much smaller than conventional implosion weapons . The W54 nuclear warhead used for special purposes and the Davy Crockett nuclear artillery unit was about 11 inches diameter and weighs 51 pounds . The W48 is 6 inches in diameter and weighs over twice as much , and probably requires twice as much plutonium . Independent researchers have determined that one model of US conventional implosion fission weapon cost $ 1 . 25 million per unit produced , of which $ 0 . 25 million was the total cost for all non nuclear components and $ 1 million the cost of the plutonium . Linear implosion weapons , requiring 2 - 3 times more plutonium , are proportionally more expensive . References . On page 10 ( 13 in the PDF ) ' A heavy spring is used to hold the pit in place ' . As there are no pits in a gun system like that of [ Little Boy ] , it is very likely the W48 was most certainly an implosion type device , and not a gun type . denotes a spherical implosion system , for an artillery piece .