decided to arm the Sheridan with just 152 mm conventional rounds until the Shillelagh was ready. The first of 12 pilots was delivered in June 1962. Pilots 1–3 comprised the first generation. With the second generation pilots 4–6, the band track was replaced with single-pin, link type tracks. An XM551 test bed turret with gun-launcher was mated to an M41 chassis, which began firing tests in August 1962 at Aberdeen Proving Ground. The third generation of pilots, starting with pilot 7, eliminated the water jet propulsion. Pilots 9–11 were delivered in 1964, and pilot 12 was delivered in February 1965. In the 1960s the Army was also developing the MBT-70 main battle tank with West Germany. The U.S. Army no longer used the heavy, medium, and light tank classifications. In 1960, with the deactivation of its last (M103) heavy tank battalion, and the fielding of the new M60 series tank, the U.S. Army had adopted a main battle tank (MBT) doctrine; a single tank filling all combat roles. The U.S. Army still retained the M41 Walker Bulldog light tank in the Army National Guard, but other than the units undergoing the transitional process, the regular army consisted of MBTs. Fearing Congress would balk at funding two developmental tank programs, the Army chose to designate the Sheridan as an armored reconnaissance vehicle rather than a light tank. The Army also believed "tank" too much evoked the main battle tank, a different role altogether, so the new project was instead officially classified as an "Armored Reconnaissance/Airborne Assault Vehicle." Production In April 1965 the Army awarded a four-year $114.5 million contract to the Cadillac Gage division of the General Motors (GM) for the production of the M109 howitzer and the XM551 General Sheridan. Limited production was approved in May, and then classified as "Standard A" in May 1966. Production took place at Cleveland Army Tank Automotive Plant. The first two production units were delivered to the Army in July. During development, the Marine Corps evaluated the Sheridan as a possible replacement for the M50 Ontos. The Corps determined that the Sheridan would be too costly. The Sheridan entered service in June 1967 with 1st Battalion, 63rd Armor Regiment at Fort Riley. 2,426 Sheridans were planned. In the end, 1,662 Sheridans were built between 1966 and November 1970. Total cost of the M551 program was $1.3 billion. The M81 gun had problems with cracks