The M40 is a recoilless rifle of US origin. It was developed in the mid 1950's to replace the unsuccessful M27 developed several years earlier. Due to its excellent firepower, ample choice of ammunition types and good range the M40 was a very successful weapon. It was widely exported and although the use of recoilless rifles has reduced drastically world wide the M40 remains in widespread use.

The M40 has a conventional design for a recoilless rifle. The M40 has a long rifled barrel, features side swiveling breech block, is attached to a tripod with a single castor wheel and is fitted with a 12.7mm spotting rifle. The M40 actually is a 105mm weapon but is designated a 106mm weapon in order to not accidentally load 105mm ammunition for the earlier M27. The M40 fires single piece ammunitions, of which many types are available.

A wide variety of ammunition types is available for the M40 and each has its own effective range and performance. HEAT rounds can be considered the standard type of ammunition and are effective up to 1.350 meters and penetrate over 400mm RHA. Explosive rounds can be used up to 2 or 3 km and APERS rounds that fire flechettes up to 300 meters. The sustained rate of fire is 1 round per minute, although a rate of fire of 5 rpm can be attained for short periods.

The M40 is generally used on a light vehicle due to its weight and due to its firing signature. However, the M40 on its tripod can be manhandled over short distances. The vehicle most often used to carry the M40 is the M151 Mutt, but a Land Rover Defender, Toyota Land Cruiser, BJ-2020 and other 4x4 vehicles are common as well. In some nations the M40 is also carried on armored personnel carriers such as the M113 or Casspir. The M50 Ontos was a US tank destroyer with 6 guns and the Japanese Type 60 had two M40's as armament.

The M40 was used by the United States military and most European nations. Most have been replaced by guided missiles, making the M40 very uncommon in Western nations. The M40 remains in widespread use in Asia, Africa, the Middle East and South America. With the introduction of anti-tank missiles the M40 is often relegated to a secondary role and nowadays is mostly used as a support weapon for infantry operations.