M40 106mm Recoilless Rifle
The M40 is an 106mm breech-loaded, single-shot, man-portable, crew-served recoilless rifle. It can be used in both anti-tank and anti-personnel roles. Recoilless rifles are capable of firing artillery-type shells at velocities, and with an accuracy, comparable to those of standard guns, but counterbalances the recoil force of firing with a portion of the thrust of the projectile. In a more conventional system, the recoil force would be counterbalanced through a combination of slides, hydraulic-mechanical devices, and/or structural supports.
The M40A1 system weighed approximately 438 pounds when assembled and mounted for firing; it has a sustained rate of fire of 6 rounds per minute and an effective range of 1365 meters. The M50A1 Ontos (Rifle Mutiply 106MM recoiless fully tracked), used by the USMC in Vietnam, mounted six of these barrels on a tracked vehicle.
The momentum manifest within a gun system during the firing of the weapon is equal and opposite to sum of the momentum imparted to the projectile launched from the gun and that of the propellant gases subsequently ejected from the gun system. This is true by Newton's third law of motion. In order to eliminate the need for heavy recoil mechanisms, some guns are built with a nozzle in the breech, so that part of the propellant gas can flow backward and counter-balance the momentum of the projectile and the part of the propellant that moves forward. Such guns are called recoilless guns or recoilless rifles.
The most desirable feature of a recoilless rifle system is its high ratio of firepower to launcher weight. The recoilless rifle makes it possible for front-line troops to launch direct-fire projectiles from a very lightweight launcher that has eliminated the cumbersome mount and recoil system formerly required. Recoilless rifle cartridges are specially designed with openings in the cartridge case to allow for the escape of gas to the rear of the weapon to counteract recoil.
In the United States, the first reduction to practice of a recoilless gun was hastily—but effectively invented—during World War II by Colonel Rene R. Studier, at the Research and Development Service of U.S. Army Ordnance, as a means to achieve a lightweight recoilless gun. The incredible result of the invention was a shoulder-fired weapon with the accuracy comparable to an Ml rifle that could propel a three-pound (1.36 kg) explosive shell with a muzzle velocity of 1200 feet per second (366 m/s). Recoilless rifle ammunition was produced in 57-mm, 90-mm, 105-mm, and 106-mm sizes.
As early as 1944, the US Army had expressed an interest in an 105mm recoilless rifle. This led to the development of the T18 and T19 recoilless rifles. Though development of these weapons stopped in the 1940s, development of the T19 was revived in 1950 because of an urgent need for more capable anti-tank weapons to support forces fighting in Korea. The T19 was type classified as the M27, but only ever intended as an interim solution.
Concurrently with the fielding of the M27, the US Army also began work on a Battalion Anti-Tank Weapon (abbreviated as BAT). The BAT was seen as a weapon that could more completely meet the requirements. This need became more pronounced as accuracy and reliability of the interim M27 weapon became apparent through experiences in Korea.
Frankford Arsenal, which had led to the projects to develop the M18, M20, and M27 recoilless rifles, was tasked supervising the initial development studies of the BAT in April 1950. In August 1950, Frankford Arsenal took on a technical supervisory role for BAT program. Design of the initial prototype barrel and chamber for the weapon, designed T136, was completed in October 1950. The chamber was fabricated using a modified version of the nozzle-breech used on the M27 recoilless rifle. By Spring 1951, the weapon had been proof fired and 3 total weapons completed. The T136, designed with a smooth bore barrel to fire fin-stablized projectiles, weighed 195 pounds, 135 pounds less than the M27.
In October 1951, the T136's chamber and barrel were modified to accept new types of ammunition also in development. Shallow rifling, of 1 turn in 360 calibers, was added to the barrel. The mounting brackets for the weapon's spotting rifle were also improved. The resulting weapon was designated as the T136E1. The new weapon weighed 197 pounds. Concerns about heat build up in the weapon during rapid firing led to structural enhancements in early 1952 that allowed for the weapon to maintain its integrity at temperatures up to 600 degrees Fahrenheit. This new variant, weighing 214 pounds, was designated as the T136E2.
During 1952, the decision was made that the new family of ammunition being developed for the BAT would include only one fin stabilized projectile, the high-explosive anti-tank round. The rest would continue to be spin-stabilized. Development subsequently switched from the spin-stabilized projectile optimized T136 series, to the T170 recoilless rifle, which had begun development at Frankford Arsenal in 1950. The T170 included modified M27 breech components that prevented the use of older visibly similar 105mm ammunition that was unsuitable for the new weapon. Various elements of the T136 series were also incorporated into the new design.
The differences between the ammunition used in the older M27 and that used in the new developmental rifles was of enough concern that the new weapon was later defined as having a bore diameter of 106 millimeters to prevent confusion. The T170 was subsequently type classified as the Rifle, 106mm, M40.
The standard M40 series was been highly successful in the military arsenals of countries, rich and poor, around the world. The M40A1 was 340 centimeters long from breech to barrel tip and weighed 209.5 kilograms in firing order. Three types of ammunition were commonly provided and they were: high-explosive anti-tank (HEAT), high-explosive squash-head (HESH), and a anti-personnel round. The HEAT round was able to penetrate up to 150 millimeters of armor at a 60 degree angle of impact. The maximum range for the M40A1 was 7,700 meters, but the effective range was around 2,750 meters with a muzzle velocity of 500 meters per second.
A total of 4 subvariants of the M40 were built (M40A1-A4), and a subvariant of the M40A1, the M40A1C was used on the M50 Ontos anti-tank vehicle. The M40 series weapons were also mounted on the M38A1C, M151C, and M825 jeeps and the M274 mechanical mule. The weapon could also be dismounted from these platforms and fired on a ground tripod. In the US Army of the 1950s fire support for the rifle company was the weapons platoon. To carry out its mission it is equipped with three 81 mm mortars and two 106 mm recoilless rifles. These weapons provided the platoon with ample firepower capable of lending full support to various missions the rifle company is assigned. The 106 mm rifles were normally employed in general support along the forward edge of the battle area, covering the most likely avenues of armor approach. If feasible, they would be so located as to afford mutual support.
The recoilless revolution had first touched the Army at the end of WWII, but it was the USMC's novel use of the 106mm weapon that nearly led to organizations larger than antitank platoons and sections. The relatively inexpensive ONTOS system mounted two to six 106mm recoilless rifles on a light tank chassis. Project Vista envisioned it as the primary antitank weapon for the Army and USMC. As the ONTOS proved unwieldy, the tank's next challenger was a first generation guided missile known as DART.
This weapon had been in use since the beginning of the Korean War, and was used by the US Marine Corps in Vietnam. The 1968 attack on the Lang Vei Special Forces base was the first to include the use of armor by the North. North Vietnam possessed Soviet furnished PT-76 amphibious light tanks, T-54, and T-59 Main Battle Tanks (MBTs); however, they had never before utilized their armor in South Vietnam. Eleven, North Vietnamese Army (NVA) PT-76’s, attacked a Special Forces base at Lang Vei in 1968, breaching the wire in support of the infantry assault. The Special Forces defenders, who possessed no armor assets, were forced to engage the NVA tanks with 64mm Light Antitank Weapon (LAW) anti tank rockets, CAS and artillery fire, and 106mm recoilless rifle fire. In fighting in Hue in 1968, the Ontos, a light tank armed with six external 106mm recoilless rifles, proved to be perfect for fighting in the confined streets of the ancient city, providing large firepower in a small package.
During the 1965 intervention in the Domincan Republic, the use of force in response to rebel attacks was limited to individual and light crew-served weapons, with specific permission required before troops could respond with recoilless rifles, bazookas, or artillery. At times this proved frustrating for American soldiers subject to sniper attack. Soon after they entered the city, they discovered that the 57mm Light Anti-Tank Weapon, and both the 90mm and 106mm recoilless rifles wre excellent anti-sniper weapons, although they did cause considerable damage in urban areas. One I06mm recoilless rifle crew also found a completely new application for the anti-tank weapon-anti-ship. After receiving permission to return fire on a rebel gun boat that shelled their position with mortar fire, the 106mm crew sank the offender with a single round.
Their back-blast constitutes a hazard to their own troops and makes them conspicuous when they fire. Because of the inefficient interior ballistics. Recoilless guns require at least twice as much propellant as conventional guns ofsimilar performance and their ammunition is, therefore, bulky and heavy. The inefficiency is caused primarily by the substantial application of chemical energy, released during combustion, to the rearward kinetic energy of the propellant gases vented through the nozzle.
The highest impulse noise producing weapon which was standard in the Army is the 106mm recoilless rifle which produced a peak pressure level of 188 dB and an estimated B-duration of 20 msec at the operator's ear. Prior to the mid-1970s, when less emphasis was placed on hearing conservation and the proper use of hearing protection, the 106mm RR was routinely fired without protection. Although it has been documented that artillerymen incurred "clinically significant hearing losses" after several years in artillery, there is no documentation indicating that any single exposure as the operator's position of a weapon such as the 106mm RR was of itself sufficiently severe to have been reported as the cause of a known permanent hearing loss or acoustic trauma.
Armor experiences early in the Korean War clearly demonstrated the need for an improved antitank weapon of minimum size and weight that would be capable of defeating the heaviest known enemy armor with pinpoint accuracy. There were a number of conventional weapons in combat use, namely, the rifle grenade with a range of 100 to 200 yards; the improved 3.5-inch Bazooka with a 300-yard range; and the 90 and 106mm recoilless rifles with ranges effective to 1,000 and 1,500 yards, respectively. However, their accurate range of application was limited to about 1,500 yards and their lethality was limited by projectile size. The most logical approach to overcoming these tactical deficiencies was the development of an antitank guided missile.
While the recoilless rifle systems are primarily for antitank purposes, antipersonnel capabilities have also been included. The projectile design is similar to artillery applications. With HEAT projectiles a fin-stabilized projectile with low spin is used. Fixed and folding fins are both used. Pre-engraved rotating bands or plastic bands are used to minimize engraving forces and pressure buildup during "shot-start". The cartridge case is of steel or aluminum and contains either circular perforations in the sidewalls or a rupture disc in the base to allow the gas to escape. A liner of cloth, paper, plastic, or a combination of rayon and plastic inside of the cartridge case protects and retains the propellant.
Existing antitank weapons, EETAC and the 106mm Recoilless Rifle, did not meet current and anticipated infantry requirements for a suitable antitank / assault capability. The reputed maximum effective range of the recoilless rifle, 1100 meters, was insufficient, signature effects were excessive; and ballistic mismatch of the spotting and major caliber rounds resulted in inaccurate fire at the greater ranges.
The TOW (Tube-launched Optically-tracked Wire-guided) Heavy Antitank/Assault Weapon (HAW) system now belongs to a family of advanced systems designed t o cope with the enemy armor threat through the early 1980's. The TOW/HAW system became operational in September 1970, initially replacing the 106mm recoilless rifle and French ENTAC system, and later the helicopter adaptation of the French SS-11 system.
The TOW ground system was programmed to replace the wire-guided ENTAC antitank missile system and the M40A1 106mm recoilless rifle on a one-for-one basis. In anticipation of this, the Army had completed phaseout of the ENTAC when the TOW was standardized on 30 September 1970. Among the benefits of TOW over the M40A1 106mm recoilless rifle were a reduction in gunner error, simplified gunner training, and a greatly increased hit capability against moving targets at all ranges from 65 to 3,000 meters. Other benefits included an increased maximum effective range from 1,100 to 3,000 meters, and a reduction of several hundred pounds in launcher system weight.
In September 1970, 3 BGM-71 TOW training battalions became operational in the US Army, and the M40 106mm recoilless rifles and the French ENTAC missile (MGM-32) began to be replaced. By 30 September 1970, the Army had completed phase out of the MGM-32 ENTAC after the BGM-71 TOW missile was standardized. The M40 106mm recoilless rifles were redistributed to the reserves and depot stocks as they were replaced by the BGM-71 TOW.
The M40 series continued to serve around the world. Iran’s state-owned Defense Industry Organization manufactures what was originally a licensed-version of the M40 called the “Anti-Tank Gun 106”. The war in Lebanon can be divided into three phases-the domestic conflict phase (spring to fall 1975), the pre-Syrian phase (fall 1975-June 1976), and the Syrian phase (June 1976-1990). Jeep-mounted 106mm recoilless rifles were used extensively by both sides against a variety of targets, including armor, buildings, and barricades, as well as in an indirect role. The Soviet man-portable AT rockets, RPG-6 and RPG-7, were found to be extremely useful both against armor, as they were designed to be employed, and against barricades and walls where they served as portable artillery. Valued as multipurpose weapons, the 106mm recoilless rifle and its Soviet counterpart, the B-l0, were used extensively to make holes in walls. Nearly three decades later, M40s came into the hands of rebels in Libya and Syria.
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