Due to the layout of the vehicle, the missile did not come into the sight of the gun/tracker system until it was 800 yards (730 m) from the vehicle, at which point it could start to be guided. In addition the missile proved to have a very long minimum range. Typical loads consisted of only 8 missiles and twenty M409 HEAT rounds for short-range use. The Shillelagh was considerably larger than a conventional round, so only a small number could be carried. These problems, in combination with the lack of suitable targets, resulted in the Sheridan's deployment to South Vietnam without the complex missile system. Firing the gun caused such a large recoil as to result in failures in the delicate missile firing electronics on the tank. They were also liable to cook off if the vehicle were hit. The combustible casings of the 152mm caseless ammunition rounds did not burn completely, requiring a complicated and slow gas-driven scavenging system. The Sheridans' 152mm main guns were used in combat operations in Vietnam but proved troublesome. Shillelagh missiles did not prove to be a problem in the Vietnam War: they were not used. In 1968 152mm main gun ammo became available, and the M551 General Sheridan was deployed to South Vietnam for combat operations in January 1969. In 1966 the US Army began pressing General Westmoreland to field the tank in South Vietnam, but he declined, stating that with no main gun ammunition, the Sheridan was basically nothing more than a $300,000 machine gun platform. The Sheridan was a light aluminum-armored AFV designed to be air transportable and provide antitank support for airborne forces. The M81/MGM-51 was first installed on the M551 Sheridan.

Mgm 51 shillelagh