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 battalions. 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.