M18 recoilless rifle

Origins and development

During World War II, the U.S. Army's Artillery Section was working on a 105 mm recoilless cannon, based on captured German models that used a plastic blow out plug in the cartridge case. At the same time, there was a freelance research by the U.S. Army's Infantry Section of a man-portable recoilless 57 mm cannon by two engineers, named Kroger and Musser. Instead of a blowout plug, the infantry section's recoilless cannon used a British development in which the cartridge case had hundreds of small holes in the side walls with a lining of plastic on the inside of the cartridge case walls to keep water and other elements out until the round was fired. Another unique innovation was the use of pre-engraving bands on the 57 mm projectile that engaged the barrel's rifling. The belief was this feature would reduce friction on firing, allowing more of the propellant gases to be used to force the shell towards the target and less being used to achieve the recoilless effect and therefore giving their design a much higher muzzle velocity than most recoilless cannon at that time period had achieved.

The "Kromuskit", as the new 57 mm weapon was called (a word play on the engineers' family names) was officially designated the T15 and first tested in November 1943. The tests proved that the Infantry Section's concept for a recoilless weapon was superior to the Artillery Section's concept and the development of the 105 mm weapon was canceled. In late 1944, the T15 was redesignated the M18 57 mm Recoilless. The cannon and 57 mm ammunition were placed in mass production. Four types of ammunition were initially produced: an anti-tank HEAT round (T20E2 / M307), an HE round (T22 / M306), a Smoke (White Phosphorus) "bursting smoke" round (T23 / M308), and a Training Practice round. By early 1945, over 2,000 M18 recoilless rifles and 800,000 rounds of ammunition were on order. After World War II ended, a canister round (T25E5) with a range of 175 meters was also produced.

US service

The weapon was crewed by a two-man team, the gunner and the loader, who fired it from a prone or kneeling position. It could also be awkwardly carried, fired from the shoulder and reloaded by one man in an emergency, fired prone from the extended T3 monopod and bipod, or fired from a fixed position on a cradle mounted on the M1917A1 machine gun tripod. The weapon was carried in a T27 Cover with two padded shoulder straps, designed to be simultaneously carried by two men in line with the straps slung over their shoulder on one side.

Ammunition was packed four shells to a wooden crate, each crate weighing about 40 lbs and had a volume of 0.86 cubic feet. Three 57mm Recoilless Rifle shells could be carried per M6 Rocket Bag and slung on one shoulder by the ammunition bearers assigned to the weapon.

World War II

The first fifty production M18 57 mm cannons and ammunition were rushed from the factories to the European Theater in March 1945. Further examples were subsequently sent to the Pacific Theater. The first combat the new cannon saw was with the U.S. Army's 17th Airborne Division near Essen, Germany. While the performance of the high explosive (HE) warhead was impressive, the M18's 57 mm HEAT round was slightly less so, only about 76.2 mm (3in.) of armor penetration at 90 degrees, compared with the M6A3 rocket for the Bazooka which had a penetration of 100 mm

In the Pacific Theater, the new lightweight 57 mm cannon was an absolute success as "pocket artillery" for the soldiers of U.S. Army infantry units that were issued the M18. It was first used in the Pacific Theater during the Battle of Okinawa on June 9, 1945, and proved with its HE and WP rounds it was the perfect weapon for the hard fighting that took place against the dug-in Japanese in the hills of that island. The only complaint the U.S. Army had was the lack of sufficient 57 mm ammunition for the M18.

Korean War

Each U.S. rifle company in the Korean War was authorized three M18 recoilless rifles. Veterans of the Korean War have mentioned the use of the M18 against enemy machine gun nests. For anti-tank tasks, however, the M18 was too weak; the Soviet-built T-34 tank was extremely hard to penetrate even for the 2.36 inch (60 mm) Bazooka of World War II. The only way would have been flank or rear shots. U.S. infantry, initially with very few capabilities against these targets, solved the problem with new weapons like the 3.5 inch (89 mm) M20 Super Bazooka, which was powerful enough to destroy T-34s.

Vietnam War

Although obsolete as an anti-tank weapon, the M18 was still used by ARVN and its allied forces in an anti-personnel role. It was able to use the NATO-standard M74 tripod.

US Navy Swift Boats used paired mounts containing triple-mounted M18s and a .50 M2HB Machine gun on the bow. It was used for fire support or demolishing obstacles.

Production and use outside the US

The M18 was copied by the People's Republic of China as the Type 36. The U.S. had provided Nationalist China the blueprints for the weapon. When the communist Chinese seized the factory, they took advantage of the facilities and blueprints to make their own copy of the weapon. China provided the communist Vietnamese the Type 36 in 1963 for use in the Vietnam War. Tanzania was also a user of the Type 36. In a strange twist, the Chinese version of the M18 can fire both U.S. and Chinese manufactured ammunition, but U.S. made M18s cannot fire Chinese ammunition. This was done deliberately by increasing the bore of their weapons and ammunition by 1mm, making their munitions too large for use by NATO weapons in the event of capture, but not vice versa.

As late as 1984, the M18 57 mm recoilless rifle was still being produced under license in Brazil by Hydroar in São Paulo.