The M40 recoilless rifle was a 105 mm weapon mislabeled as 106 mm caliber.

Wars & Conflicts · Published 15 October 2026 · Updated 25 October 2026 Swipe more like this →

In short: Designed to fix the reliability flaws of its predecessor, the M40 became a global staple of infantry warfare. While its primary role was anti-tank combat, this versatile tube-launched weapon found its way onto everything from jeeps to coastal patrol boats, and even served a surprising secondary life in avalanche control.

The M40 recoilless rifle was developed by the U.S. Army in 1955 to replace the M27, a rushed design fielded during the Korean War. The M27 suffered from poor reliability due to rear-mounted trunnions and lacked an effective sighting system. To distinguish the new, improved M40 from the failed M27 and its incompatible ammunition, the Army designated the M40 as a 106 mm weapon, despite its bore actually measuring 105 mm. This air-cooled, breech-loaded rifle fires fixed, one-piece cartridges, with propellant gases vented through the rear to neutralize recoil.

A defining feature of the M40 is the M8C .50 caliber spotting rifle mounted on top. This rifle fires a specialized 12.7x77mm round that mimics the trajectory of the main 105 mm projectile, creating a puff of smoke upon impact to help the gunner adjust their aim. While the spotting rifle could theoretically be used against personnel, U.S. military doctrine discouraged this to keep the M40's position hidden. This tactical restriction is widely believed to be the origin of the persistent myth that international law prohibits using .50-caliber weapons against enemy soldiers.

The M40’s adaptability allowed it to be mounted on a vast array of platforms, including the M50 Ontos—a vehicle carrying six of the rifles—and various light utility vehicles. Though largely replaced by the BGM-71 TOW missile system in the U.S. military, the M40 remains in service globally. Its civilian career was equally notable; the U.S. Forest Service used the rifles to trigger controlled avalanches. However, after a fatal 1995 accident and subsequent catastrophic bore explosions in 2002, the Forest Service retired the weapon in 2003, marking the end of its unconventional role in mountain safety.

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