As more and more stout Soviet-originated tanks permeated possible battlefields of the Cold War, the United States military funded many anti-tank weapon ventures. One chief development emerging from the later stages of World War 2 (1939-1945) was the recoilless rifle, a relatively lightweight weapon attempting to counter violent recoil forces by jettisoning some of the escaping propellant gasses to the rear while the projectile exited the gun tube at front. Many tank counters were eventually designed around this basic principle including the classic world renown Carl Gustav launcher from Sweden (detailed elsewhere on this site).
The M40 entered service with American forces in the middle of the 1950s, missing out on service in the Korean War (1950-1953) but was heavily featured in the Vietnam War (1955-1975) that followed. Since then, the weapon has gone on to see considerable action in conflicts like the Algerian Civil War (1954-1962), the Indo-Pak War (1971), the Iran-Iraq War (1980-1988) and - more recently - in the Libyan Civil War (2011) and the ongoing Syrian Civil War (2011). Its overall simplicity and low operating / procurement costs have made it a battlefield fixture despite its 1950s origins (particularly favored by modern-day rebel forces like those in the Syrian conflict).
Manufacture of the guns stemmed from the Watervliet Arsenal of Watervliet, New York. The weapon was developed from the largely M27 105mm gun series which gave less-than-stellar performance for its time as a frontline weapon. It saw most of its service in the Korean War and was remembered for its excessive handling weight and general reliability issues (the latter largely due to its hurried development phase). Additional work on the line produced the "T136" developmental designation until finalization begat the "M40". The new gun was, therefore, an improved form of the original M27 which had its own roots in the latter stages of World War 2.
The M40 was designed to fire a fixed projectile arriving in several flavors: High-Explosive, Anti-Tank (HEAT), HEAP (High-Explosive, Anti-Personnel), APERS (flechette) and Canister shot (among others). The gun was chambered to fire a 105x607mmR projectile which helped differentiate it from the earlier M27 recoilless rifle (this weapon's 105mm projectiles were not compatible with the M40). Beyond its usefulness in the anti-tank/anti-armor role, the M40 could be used against dug-in enemy infantry through special anti-personnel rounds issued and as a "bunker buster" when attempting to tackle hardened structures. The weapon could be featured on its standard tripod assembly or mounted atop a vehicle for a fast, hard-hitting assault approach. The original projectile held value against armor some 400mm thick.
As a complete system, the M40 weighed 462 lb and featured an overall length of 11 feet. A heavy-duty tripod supported the weapon through all facets of its operation (unless vehicle-mounted). The operating crew numbered at least two personnel and loading/reloading of the shells was through the breech-end of the tube. A sighting device allowed for accurized fire at range but engagement angles were strictly limited to Direct-Line-of-Sight (DLOS). An M8 .50 caliber rifle was seated above the tube for spotting purposes. The tripod gave an inherent -17 to +65 degree of elevation reach as well as full 360-degree traversal left or right of centerline. Muzzle velocity of the exiting shells was 1,650 feet per second and effective ranges were out to 1,480 meters. Maximum range was listed at 6,870 meters.
The M40 series went on to see considerable export sale / usage in its service years. Operators ranged from Australian and Austria to Uruguay and Venezuela. Some later, foreign-developed projectiles gave increased penetration of armor up to 700mm thick. The M40 recoilless rifle was the featured weapon on the American M50 "Ontos" tracked vehicle (detailed elsewhere on this site) and six such weapons were installed on the compact tank (though all had to be reloaded externally). The Japanese Self-Defense Force (JSDF) developed the "Type 60" tank destroyer which seated two M40 guns in a side-by-side arrangement over the right side of the hull. Iranian defense industry produced the M40 locally as the "Anti-Tank Gun 106".
Many M40 systems have been retired by fist-rate armies of the world particularly with the rise in effectiveness of Anti-Tank Guided Missiles (ATGMs). The U.S. Army adopted the Hughes BGM-71 TOW anti-tank missile series to succeed weapons like the M40. The BGM-71 series appeared in 1970.
Production of the M40 has been vast with participants found (whether through legal or illegal production means) in Austria (Lohner GmbH), China (NORINCO), India (Ordnance Factory Board), Iran (Defense Industries Organization), Pakistan (Pakistan Machine Tool Factory Ltd), South Korea (KIA Machine Tool Company), and Spain (Santa Barbara Sistemas).