The Ford MGM-51 Shillelagh was an American anti-tank guided missile designed to be launched from a conventional gun (cannon). It was originally intended to be the medium-range portion of a short, medium, long-range system for armored fighting vehicles in the 1960s and '70s to defeat future armor without an excessively large gun. Developing a system that could fire both shells and missiles reliably would prove complex and largely unworkable. It served most notably as a primary weapon of the M551 Sheridan light tank, but the missile system was deleted from units serving in Vietnam. Ultimately very few of the 88,000 rounds produced were ever fired in combat.
The Shillelagh was a disappointment compared to the later BGM-71 TOW anti-tank guided missile first produced in 1970 by the U.S. The TOW system, which could not fire gun rounds, and was guided by a wire which directly sent commands to the missile, would prove simpler and more versatile. TOW would become the most widely used anti-tank guided missile in the world based on a range of light, armoured and flying vehicles. Main battle tanks of the late 20th century such as the successful M1 Abrams tank would field improved conventional 105mm and 120mm guns which proved effective against enemy armor threats. While the Russians are developing gun launched missiles, the US and NATO are developing guided tank shells.
The name comes from a type of wooden club associated with Ireland.
With the rapid increase in armor during World War II, tanks were becoming increasingly able to survive rounds fired from even the largest of WWII-era anti-tank guns. A new generation of guns, notably the British 105 mm Royal Ordnance L7, were able to cope with newer tanks, but it appeared that in another generation the guns needed would be too large to be practical.
Instead the US Army started concentrating on HEAT rounds in the 1950s. HEAT penetration is not dependent on the speed of the round, allowing it to be fired at much lower velocities, and thus from a much lighter gun. They also work better at larger diameters, and a large-diameter low-velocity gun makes for an excellent assault gun vehicle. On the downside, the low speed also means that they become increasingly hard to aim over longer distances. The US Army looked to address this problem with the use of HEAT-equipped guided missiles for anything beyond a few hundred yards.
In 1958 they felt the state of the art had progressed enough to start work on such designs, and in June 1959 Sperry and Ford Aeronutronic were asked for designs to fill the shorter range role. Ford won the contract and started work on the XM13. The first test shots occurred in 1960, and limited production started in 1964, now known as the MGM-51A.
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