of the M41 was fairly short; at 25 tons it was considered too heavy to be a true light tank, and had a rather short cruising range. Plans were started to build an even lighter replacement mounting the same gun, the T71 and T92. The T92 appeared to be the more promising of the two. However, as the prototypes were entering testing, information about the new Soviet PT-76 light tank became available. The PT-76 was amphibious, and soon there were demands that any U.S. light tank should be able to swim as well. The T92 could not be easily refitted for this role, so the Army canceled the program in 1958. In January 1959, the first concept studies were initiated for the armored reconnaissance/airborne assault vehicle that would replace both the M41 and M56 Scorpion self-propelled gun. By October 1959, 12 proposals had been received by the Ordnance Tank Automotive Command. Two proposals were downselected in December: One from Cadillac Motor Car Division of General Motors Corporation, and a joint venture of AAI Corporation and Allis-Chalmers Manufacturing Company. Mockups of both proposals were evaluated in May 1960. The AAI candidate had three crewmen, and weighed the closest to the 10-ton weight limit specified in the requirements. The Cadillac design was only slightly heavier, with four crew. The three-man turret of the Cadillac proposal was considered more effective than the two-man turret proposed by AAI. The weight limit was reset at 15 tons. In June 1960, Cadillac Motor Car Division signed a contract to develop their concept further, which was designated as the AR/AAV XM551. In August 1961, the Secretary of the Army approved the name "Sheridan," after Major General Philip Sheridan. A test bed underwent operations at Cleveland Tank Plant in December 1961. The decision to use the 152 mm caliber XM81 gun-launcher instead of a more conventional gun was driven by the desire to save weight. The XM81 weighed about half as much as the 105 mm caliber M68, and could fire both conventional and missile rounds. Testing of the XM81 began at Erie Army Depot in late 1961. By 1962, the Army realized that the MGM-51 Shillelagh missile system would not immediately be ready for the Sheridan, and so considered several alternative gun systems. These including conventional 76 mm, 90 mm and 105 mm options, as well as missile systems such as ENTAC and TOW. The Army ultimately