The M109 was the medium variant of a U.S. program to adopt a common chassis for its self-propelled artillery units. The light version, the M108 Howitzer, was phased out during the Vietnam War, but many were rebuilt as M109s.

The M109 was part of a U.S. program that aimed to adopt a common chassis for self-propelled artillery units. The light variant, the M108 Howitzer, was phased out during the Vietnam War, but many of them were rebuilt as M109s. The Ground System Division of United Defense LP, now known as BAE Systems Land and Armaments, developed the M109. Interestingly, the name "Paladin" comes from the lead character in the early TV western "Have Gun – Will Travel."

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