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Specifications (M109A2)
Engine: 405 hp Detroit Diesel 8V71T Model 7083-7396 two-stroke eight-cylinder
Transmission: Allison XTG-411-A2 cross-drive with four forward and two reverse speeds
Length: 29 feet 11 inches
Width: 10 feet 4 inches
Height: 10 feet 9 inches
Ground clearance: 18 inches
Combat weight: 55,000 pounds
Armament: One M185 39-caliber 155 mm howitzer with 36 rounds of ammunition, one flexible turret-mounted M2HB .50-caliber machine gun with 500 rounds
Emplacement time: One minute
Rate of fire: Four rounds per minute for three minutes; one round per minute sustained
Maximum effective range: 11.2 miles (14.6 miles rocket-assisted)
Gun depression/elevation: -3 to 75 degrees
   Road: 35 mph
   Cross-country: 12 mph
Fuel capacity: 135 gallons internal
Maximum range: 217 miles
Crew: Six

The M109 was conceived in the early 1960s by the Ground Systems Division of United Defense (now part of BAE Systems) as a replacement for the M44 self-propelled howitzer using a chassis and other components common to other U.S. armored vehicles. Accepted in November 1962 and entering service in Vietnam, where its 360-degree traverse made it a welcome asset at firebases, the air-transportable M109 mounted an M126 23-caliber 155 mm howitzer within an armored turret, as well as a flexible .50-caliber M2HB machine gun. A lighter version, the M108, with a 105 mm howitzer, entered service in 1966, but was abandoned in 1975, many being rebuilt to carry the M109’s 155 mm gun.

In 1973 the M109A1 entered production, featuring a M189 39-caliber 155 mm cannon with a longer tube and more efficient muzzle brake, which increased its range from 9 miles to 11.2 miles. A succession of upgrades in armament and technology followed. Introduced in 1991, the M109A6 Paladin featured an advanced fire control system that allowed its gun to fire with accuracy within 30 seconds of the vehicle coming to a stop. The latest model is the M109A7, introduced in 2013. Although it shares the same chassis, transmission and tracks as the Bradley fighting vehicle, the 78,000-pound M109A7 is faster and more maneuverable. Used by armies worldwide, M109s have seen combat in every American war since Vietnam, as well as the 1973 Yom Kippur War, the 1982 and 2006 Israeli interventions in Lebanon, and the 1980–88 Iran-Iraq War. They remain in active service with no sign of outliving their usefulness. MH

This article appeared in the September 2021 issue of Military History. For more stories subscribe here and visit us on Facebook.