in the medium and long-range anti-tank engagement. The muzzle velocity was so low that a HEAT round fired at longer ranges would have to be "lofted", making aiming difficult, and the flight time would be so long that a moving target would be very difficult to hit. However, it appeared there was a solution to this problem by equipping the tank with gun-fired anti-tank missiles. For longer range engagements a missile would be fired instead of a HEAT round, and although its velocity would also be relatively slow, the guidance system would make a hit highly likely. A number of vehicles mounting only ATGM's, or alternately recoilless rifles like the US's own Ontos tank were already in service, but typically these vehicles had limited firepower in the infantry support role, or in the case of Ontos, could not be reloaded from within the vehicle. The XM551 offered a compelling compromise: for infantry support, the large caliber gun allowed it to fire full-sized artillery rounds and canister shot, while also giving it reasonable short-range anti-tank performance from the same gun. Although the Shillelagh missile was considered a risky project, if it worked, the XM551 would be able to deal with even the largest tanks at extreme ranges. The vehicle designed to mount the gun had a steel turret and aluminum hull. It was powered by a large diesel engine. The M551 thus had excellent mobility, able to run at speeds up to 45 mph. Amphibious capability was provided by a flotation screen. Production started on late July 1966, and entered service in June 1967. More than 1,600 M551s were built between 1966 and 1970. Total cost of the M551 program was $1.3 billion. The vehicle proved to be very noisy and unreliable under combat conditions. The armor was thin enough that it could be penetrated even by heavy machine gun rounds as well as being highly vulnerable to mines. Firing the gun would often adversely affect the delicate electronics, which were at the early stages of transitioning to solid state, so the missile and guidance system was omitted from vehicles deployed to Vietnam. Thus, despite the production of 88,000 of the expensive missiles, few were ever used in combat. The gun had problems with cracks developing near the breech after repeated firing. Most field units were modified to help address the problem and the modified M81E1 was introduced with a shallower