M2 4 . 2 inch mortar The M2 4 . 2 inch Mortar was a U . S . rifled 4 . 2 - inch ( 107 - mm ) mortar used during the Second World War and the Korean War . It entered service in 1943 . It was nicknamed the " Goon Gun " ( from its large bullet - shaped shells , monopod , and rifled bore - like a rifle for shooting Goons ) or the " Four - Deuce " ( from its bore size in inches ) . History . The first 4 . 2 - inch mortar in U . S . service was introduced in 1928 and was designated the M1 Chemical Mortar . Development began in 1924 from the British 4 - inch ( 102 mm ) Mk I smooth - bore mortar . The addition of rifling increased the caliber to 4 . 2 - inch . The M1 fired chemical shells to a range of 2 , 195 meters . It was meant to fire smoke shells , as the postwar peace lobby opposed military spending on explosive or poison gas shells . The M2 could be disassembled into a separate mortar tube ( 105 lbs . [ 47 . 72 kg . ] ) , standard ( 53 lbs . [ 24 . 09 kg . ] ) , and baseplate ( 175 lbs . [ 79 . 54 kg . ] ) to allow it to be carried by its crew . The tube had a screw - in cap at the bottom that had a built - in fixed firing pin . The " standard " was a recoiling hydraulic monopod that could be adjusted for elevation . The baseplate had long handles on either side to make it easier to carry . Upon the entry of the United States into World War II , the U . S . Army decided to develop a high explosive round for the mortar so that it could be used in a fragmentation role against enemy personnel . In order to extend the range to 2 , 926 meters , more propellant charge was used and parts of the mortar were strengthened . Eventually , the range of the mortar was extended to 4 , 023 meters . The modified mortar was redesignated the M2 . The M2 was first used in the Sicilian Campaign , where some 35 , 000 rounds of ammunition were fired from the new weapon . Subsequently , the mortar proved to be an especially useful weapon in areas of rough terrain such as mountains and jungle , into which artillery pieces could not be moved . The M2 was gradually replaced in U . S . service from 1951 by the M30 mortar . Starting in December 1942 , the US Army experimented with self - propelled 4 . 2 - inch mortar carriers . Two pilots vehicles based on the M3A1 halftrack were built , designated 4 . 2 inch Mortar Carriers T21 and T21E1 . The program was cancelled in 1945 . Tactical organization . 4 . 2 - inch mortars were employed by Chemical mortar battalion s . Each battalion was authorized forty - eight M2 4 . 2 - inch mortars . In World War II , an infantry division was often supported by one or two chemical mortar companies with twelve mortars each . In the Korean War , an organic company of twelve 4 . 2 - Inch mortars supported each infantry regiment . Ammunition . The M2 uses ammunition that is unusual . It uses the barrel ' s rifling to stabilize it in flight ( like a bullet ) rather than using tailfins like most mortars . Such ammunition is called " spin - stabilized " . The mortar ' s M3 high explosive ( HE ) shell packed 3 . 64 kilograms of explosive charge , placing it between the M1 105 - mm HE shell ( 2 . 18 kilograms of charge ) and M102 155 - mm HE shell ( 6 . 88 kilograms of charge ) in terms of blast effect . The mortar could also fire white phosphorus - based smoke shells and mustard gas shells . Chemical weapons were not used in these wars and the U . S . ended up with a large number of these shells , declaring over 450 , 000 of them in stockpile in 1997 when the Chemical Weapons Treaty came into force . Destruction efforts to eliminate this stockpile are continuing with a few of these aged shells occasionally found to be leaking . Use by other nations . The M2 was exported by the U . S . to other nations . As of 1984 , nations using the M2 included Austria , Bolivia , Brazil , Cambodia , Colombia , Ethiopia , Greece , Guatemala , Japan ( Japan Ground Self - Defense Force ) , Laos , Tunisia , and Paraguay .