The next major revision of the M109 line occurred during the middle part of the 1980s prior to the end of the Cold War. As the threat of nuclear war had not subsided, it was thought prudent to provide M109 crews with some form of local protection from Nuclear, Biological, and Chemical (NBC) agents and, thusly, an NBC suite was added to the M109. Existing M109A2 and M109A3 marks then graduated in this fashion to become the M109A4. The upcoming M109A5 introduced the M284 series 155mm main gun which replaced the original's M185 model and a 440 horsepower diesel engine was brought along. Expectedly, maximum range of the gun was increased as was performance of the vehicle. M109A5+ signified M109A5s with updated Fire Control Systems (FCSs) and other more subtle internal changes.
After Operation Desert Storm (1991), the M109A6 "Paladin" entered service in 1992 as a wholly improved M109 mark - which many regard as the ultimate evolution of the series. The A6 model improved armor protection for the crew and systems alike and further increased ammunition stocks aboard. An automatic FCS was introduced which assisted in gun laying and improved accuracy. The NBC suite and crew comfort was further addressed while the gun received additional attention. Unassisted engagement ranges now peaked at around 18.6 miles while Rocket-Assisted Projectiles (RAPs) added additional downrange reach. By this time the crew had been reduced to four from six.
M109s were resupplied in the field with additional ammunition and charges through the M992 vehicle. This development was essentially a turret-less M109 though with a fixed superstructure and three crew. The vehicle typically followed M109s into action and supplied the needed munition types on call, the shells being passed through openings at the rear of the M109.
The M109 proved an export success on the global stage, one of the most successful armored vehicles of the Cold War and one of the best American SPG/SPG systems since the close of World War 2 (1939-1945). Operators ranged from Canada to South America, Europe and the Middle East. Chances are those nations fielding an SPG during the Cold War managed a stock of either the Soviet equivalent machine or the American-made M109. Many remain in service today - such has been the effectiveness of the vehicle series in combat.
Combat service has taken the M109 through the Vietnam campaign, the Yom Kippur War (1973), the Iran-Iraq War (1980-1988), the Gulf War (1991), the recent U.S.-led Iraq campaign (2003-2011), and the 2006 Lebanon War. The British Army relied on the M109 for a time until succeeding these units with the locally-designed and developed AS-90 system appearing in 1993.
At one point, the XM2001 "Crusader", detailed elsewhere on this site, was intended as the American successor for the M109 line. This promising program was eventually terminated in May of 1992 - signaling an extended service life for existing M109 vehicles.
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