program. In 2000, the Army instead selected a variant of the General Dynamics' LAV III, later type-classified as the M1128 Mobile Gun System. The Mobile Gun System's problematic service history led to the vehicle's planned retirement in 2022. The Army has initiated development of a light tank acquisition program called Mobile Protected Firepower. Design Armament Building a vehicle lighter than the T92 required an innovative solution for the main armament. A gun firing kinetic energy penetrators to defeat modern tanks at reasonable range was too large for the XM551; gun weight was typically dependent on caliber and muzzle velocity. This was solved by arming the XM551 with a 152 mm M81 gun firing low velocity M409 HEAT rounds. The large caliber ensured it would produce a powerful shaped charge effect capable of penetrating tank armor, since velocity has no effect on shaped charge penetration, while the short gun would reduce overall weight. The gun was ideal for infantry support. The large, low velocity gun could fire a large explosive shell or canister shot. In comparison, high-velocity anti-tank guns over-penetrated soft targets, while smaller caliber weapons could not carry as great a payload. The M81E1 was not ideal in medium- and long-range tank engagements as its low velocity produced extended flight times, and made it difficult to lead moving targets. Therefore, the gun was also designed to fire MGM-51 Shillelagh anti-tank missiles. The low launch velocity against longer-ranged targets was not an issue for the missile. However, the MGM-51 was considered a risky project. A number of existing vehicles already mounted only ATGMs, or alternately recoilless rifles like the M50 Ontos, but these typically had limited utility in the infantry support role, or in the case of Ontos could not be reloaded from within the vehicle. The XM551 appeared to offer a superior balance between anti-tank and infantry support. Mobility Tactical mobility The Sheridan was powered by a large 300-hp (224 kW) Detroit Diesel 6V53T diesel engine, and a flat track (no support rollers). The XM551 thus had an excellent power-to-weight ratio and mobility, able to run at speeds up to 72 km/h (45 mph). However, the vehicle proved to be very noisy and unreliable under combat conditions. The Sheridan could swim across a river that was about 46 m (50 yards) wide. Tanks in the Patton series (M46, M47, M48), as well as the M60 tank could not perform these