The self-propelled artillery piece the M-109, was commissioned with a specific brief from the American army, to develop a heavy howitzer with had greater mobility on all types of terrain than the dated M-44. The first M-109s entered service in the early 1960s and immediately proved their worth as a major update on the M-44s. The M-109 was also continually updated during its period of service to match evolving operational needs. The 155 mm main armament was also improved with the introduction of the A1 version with its long-barrelled M185 gun, which provided a significantly increased range of fire. The subsequent A2 version incorporated further improvements of the weapon including, aiming, ammunition and target acquisition systems. The M-109 was produced in large numbers, equipping both American armed forces and key NATO allies.