The M40 recoilless rifle was a lightweight, portable, crew-served 105 mm weapon intended primarily as an anti-tank weapon made in the United States. The weapon is commonly described as being 106 mm, but it is in fact 105 mm; the 106 mm designation was designed to prevent confusion with the incompatible 105 mm ammunition from the failed M27. It could also be employed in an antipersonnel role with the use of the antipersonnel-tracer flechette round. It can be fired primarily from a wheeled ground mount. The air-cooled, breech-loaded, single-shot rifle fired fixed ammunition. It was designed for direct firing only, and sighting equipment for this purpose was furnished with each weapon.

The M40 primarily saw action during the Vietnam War and was later replaced by the BGM-71 TOW anti-tank missile system. The weapon was also used by anti-communist forces in Angola mounted on HMMWV's.

Description
The M40 is shaped like a long tube with a 0.50 cal spotting rifle above. On the left hand side, there is an elevating wheel, in the centre of which is the trigger wheel used to fine adjust the elevation and at the same time firing the spotting rifle when pulled, and the gun when pushed. The mounting is a tripod, but the front leg has a castoring wheel. On top of the mount is a traverse wheel. On the centre of the traverse wheel is a locking wheel, when the wheel is down, the rifle is locked in traverse, and can only be moved right and left with the traverse wheel. When the wheel is raised, the rifle can be traversed by hand.

The whole mounting can be placed on a M151 Jeep for mobile use. It has also been mounted on Land Rover Defenders, M113s, Mercedes-Benz G-Wagen, HMMWVs, Toyota Land Cruisers, AIL Storms and M274 Mechanical Mules.

A special vehicle called Ontos carried 6 M40's. A version specific to the T195E5 mount, the M40A1C, was used. It was used only by the US Marine Corps. Japan produced a self propelled gun called the Type 60 which carried two side by side. Some Pakistani M113s have a dual mounting.

Ammunition
Ammunition for the 105 mm rifle was issued in complete fixed cartridges. The term "fixed" means that the projectile and the cartridge case are crimped together. This insures correct alignment of the projectile and the cartridge case. It also permits faster loading because the projectile and the cartridge case are loaded as one unit. The rear end of the cartridge case is perforated, to allow the propellant gas to escape through the vented breech, thus neutralizing recoil. The projectiles used are pre-engraved, that is, the rotating bands are cut to engage the rifled bore.

Types of ammunition included: HEAT, High Explosive Plastic-Tracer (HEP-T), and Anti-Personnel-Tracer.

The ammunition for the 0.50 cal spotting rifle is not .50 BMG. The round used is a special round designed to simulate the flight path of the 105 mm ammunition.

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