Recoilless rifles are a specialized category of battlefield weaponry that see less service today than in the years following World War 2 and the arrival of guided missile systems in the 1960s and 1970s. In their most basic form, the recoilless rifle allowed a gunnery team the capability of firing a large-caliber artillery-type projectile at a line-of-sight target, the weapon using the inherent propellant gasses generated by the exiting projectile to reduce recoil. The shell was, therefore, completed as a single-piece encased component containing both the warhead and required propellant. The projectile was loaded conventionally through the breech of the weapon and the barrel was rifled to provide a rotational effect upon the exiting projectile (as opposed to a "smoothbore" weapon such as a mortar). The igniting propellant was directed away from the projectile and towards the rear of the breech when the weapon was fired, escaping through an opening. In effect, the exiting propellant gasses served to counter the exiting projectile, resulting in acceptable or near-zero recoil for a weapon of this caliber (when compared to conventional artillery systems). As such, the recoilless rifle did not need to be fielded with a complex recoil reduction system (called a recoil dampener) which led to a relatively "lighter" overall system weight. While recoil was not completely dampened in the recoilless rifle, the method allowed for infantry elements to field their own portable anti-tank capability. Recoilless rifles were also useful in the infantry support role where they could use high-explosive shells against infantry to good effect. One of the most popular recoilless rifles ever produced became the Cold War-era 84mm "Carl Gustav" launcher which still sees regular service around the world today. However, guided anti-tank missiles have largely replaced the recoilless rifle in the most modern of armies.
The M20 sported a running length of 6 feet, 10 inches and a weight of 114.5lbs. Despite these numbers, the M20 was intended as a portable battlefield weapon system. Its design was rather straightforward with the barrel being the largest component the weapon. It was smooth in appearance and lacked any muzzle brake at the business end. There were carrying handles along the sides of the barrel system just aft of the midway point. The rear was identified by the enlarged conical exhaust section tapered outwards to which a vented screen cap was affixed to allow for the exiting of propellant gasses needed in the firing action. The shape of this assembly proved critical in containing the spent gasses and directing them to the rear of the weapon. The barrel typically sat atop a heavy duty tripod assembly for support, usually the M1917A1 machine gun mount, but could be mounted to vehicles as needed. The tripod was installed well aft of the weapon's center point as the weight was generally concentrated towards the rear of the weapon as a whole. The tripod was adjustable for height and elevation and allowed for 360-degree traversal while a sighting device was affixed to the left side of the barrel.
The M20 made use of a 75x408mm R (2.95") projectile that was identifiable by its largely perforated base which allowed propellant gasses to quickly escape towards the vented cap at the rear of the breech. Nearly half of the projectile was perforated in this fashion with the warhead being conventionally conical in shape. The 75mm projectile was loaded through the rear of the breech which held a two-handled vented door for quick access. The door was then closed and the weapon made ready to fire. The barrel of the M20 was rifled with a uniformed right-hand twist design which produced the needed rotation of the exiting projectile upon launch (hence no stabilization fins seen on the projectile itself). Muzzle velocity was equal to 1,000 feet per second using the standard 20.5lb HEAT (High-Explosive, Anti-Tank) round which gave a range out to 7,000 yards in ideal conditions and armor penetration up to 100mm (4"). Other available projectile types included a basic 21.8lb High-Explosive round for countering soft targets and infantry and a 22.6lb smoke round useful in generating smoke screens. Due to the nature of the M20 and its associated weights, a typical gunnery crew numbered at least two to three personnel. Of course it was required that no personnel were positioned to the rear of the weapon for obvious reasons concerning the exiting propellant gasses.
The US Army's move towards fielding capable recoilless rifles finally produced viable forms by the end of World War 2. In the latter months of the conflict, the M20 was pressed into service and shipped in limited quantities beginning in March of 1945 to American forces in both the European and Pacific Theaters of War. Their use was just as limited for the war in Europe was over in May and the war in the Pacific in August.
With M20 stocks having grown since the close of World War 2, the M20 was pressed back into action with the arrival of the Korean War after the Communist North invaded the Democratic South. The US-led UN sought to contain the spread of communism by any means included armed deterrence. While the M20's armor-piercing qualities left much to be desired, particularly when tackling the fabled Soviet T-34 Medium Tanks fielded by the North and Chinese, the system excelled against light armored targets and fixed infantry defensive positions. As a result, it served well in the short-ranged infantry support role moreso than the intended anti-tank role.
The M20 saw some limited action in the early years of the Vietnam War but generally fell out of favor with the arrival of wire-guided anti-tank missile systems now coming online. Despite its World War 2 pedigree, the M20 still sees some limited frontline service in inventories of less-developed armies around the world.