The M55 Self-Propelled Howitzer was an American development emerging out of the 1950s. The vehicle was designed using existing M46 and M47 Patton series of medium combat tanks which heavily stocked US Cold War inventories at the time. The M55 was similar in scope and function to the preceding M53 self-propelled assault gun armed with a 155mm main gun barrel, but the M55 utilized a much larger 203.2mm M47 howitzer instead which limited total ammunition carried due to the heftier shells. Production of M55s was handled by the Pacific Car & Foundry Company throughout the 1950s. Many US Army M53 systems were later upgraded to the M55 standard. Some M55s were utilized operational during the American involvement in the Vietnam War (1955-1975) though they were eventually removed from service.