The M40 recoilless rifle is a portable, crew-served 105 mm recoilless rifle made in the United States. Intended primarily as an anti-tank weapon, it could also be employed in an antipersonnel role with the use of an antipersonnel-tracer flechette round. The bore was commonly described as being 106 mm caliber but is in fact 105 mm; the 106 mm designation was intended to prevent confusion with incompatible 105 mm ammunition from the failed M27. The air-cooled, breech-loaded, single-shot rifle fired fixed ammunition and was used primarily from a wheeled ground mount. It was designed for direct firing only, and sighting equipment for this purpose was furnished with each weapon, including an affixed spotting rifle. A limited number of M50 Ontos were built as a self-propelled light armored tracked anti-tank vehicle. They had six 106 mm M40 recoilless rifles as its main armament, which could be fired in rapid succession against a single target to guarantee a kill. Replacing the M27 recoilless rifle, the M40 primarily saw action during the Vietnam War and was widely used during various conflicts thereafter in Africa or in the Middle East. It was replaced by the BGM-71 TOW anti-tank missile system in the US Armed Forces. Design history The earlier M27 recoilless rifle was a 105-mm weapon developed in the early 1950s and fielded in the Korean War. Although a recoilless rifle of this caliber had been a concept since the Second World War, the weapon was hurriedly produced with the onset of the Korean War. The speed with which it was developed and fielded resulted in problems with reliability caused by trunnions that were mounted too far to the rear. The M27 was also considered too heavy by the U.S. Army and had a disappointing effective range due to the lack of a spotting rifle. Taking the M27 as the basis for a new design, the Army developed an improved version of the M27 that was type-designated the M40 106-mm recoilless rifle in 1955. Although unsuitable for military purposes, M27 recoilless rifles were used to trigger controlled avalanches at ski resorts and mountain passes in the United States. Description The M40 is shaped like a long tube with an M8C .50 cal spotting rifle above. The spotting rifle fires a round whose trajectory closely matches that of the 105 mm round and gives off a puff of smoke on impact with the target. On