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Further Reading

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.

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 weighs 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 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.