It was armed with the technically advanced but troublesome M81/M81 Modified/M81E1 152 mm gun/launcher, which fired both conventional ammunition and the MGM-51 Shillelagh guided anti-tank missile.

Based on its experiences in Vietnam, the Army realized the shortcomings of the Sheridan, and after the war in 1975 began to eliminate the vehicle from its units in 1979.

[5] In the immediate post-World War II era, the U.S. Army introduced the M41 Walker Bulldog into service to fill the role of a light tank.

[27] A GAO report leaked in May 1969 revealed the Army had fast-tracked the program to avoid budgetary scrutiny, despite indications by May 1966 that the tank's caseless ammunition was prone to cooking off.

The problem had since then been resolved by a compressed-air system that forced hot ammunition residue from the breech, the Army told Congress.

The report attributed several Vietnam War casualties to Sheridan design faults, and said that the tank had been wholly unready for combat there "without extensive and costly retrofits.

[34] By the end of its combat debut in 1972, the Sheridan had seen extensive action in the Vietnam War, being assigned to nearly all armored cavalry squadrons involved in that conflict.

[36] In addition to the problems presented by aluminum construction, the Sheridan had a defect that no other common armored vehicle possessed: it fired combustible-cased 152 mm main gun rounds.

The white/silver-colored bag had a strap attached to the bottom, which the loader would grab and pull off prior to gently inserting the round into the breech.

Once a mine or RPG-type weapon created an ignition source, smoke and fire became imminent, and it became a matter of Standing Operating Procedure to abandon the tank immediately.

[38] On 15 February 1969, just one month after the Sheridan's arrival in South Vietnam, an M551 from the 3/4th Cavalry detonated a 25-pound pressure-activated land mine, which ruptured its hull and ignited the combustible-case charges of the 152 mm rounds, resulting in a secondary explosion that destroyed the tank.

Anti-mine armor kits composed of spaced aluminum and steel plates were applied to the hull bottom, although only covering from the front to halfway to the end.

In the early 1980s, the M551A1 was fitted with a visual modification kit to resemble Warsaw Pact vehicles from Soviet BMP-1, ZSU-23, T-55 to T-80s, at the National Training Center at Fort Irwin, California.

[5] The OPFOR Sheridans no longer fulfill that role, having been retired at the end of 2003 and subsequently scrapped or made available as "hard targets" or, in a few cases, as museum pieces.

The Sheridan's only air drop in combat occurred during the United States invasion of Panama (Operation Just Cause) in 1989, when 14 M551s were deployed with C Company, 3/73rd Armor, 82nd Airborne Division.

[46] Four M551s were secretly delivered to Howard Air Force Base in Panama aboard a C-5 Galaxy in November 1989, days before the invasion.

These Sheridans took part in the attack on the Comandancia, initially supported by fire from Quarry Heights, and later displacing forward into the city.

As part of Team Armor, these Sheridans later provided support to JSOC elements as they secured high-value targets throughout Panama City.

[48] In the early days of Desert Shield, Sheridans were airlanded in Saudi Arabia before coalition heavy armor arrived by ship.

Other than some overheating problems in the summer months of the buildup,[52] the Sheridan suffered no mechanical breakdowns in combat and performed extremely well.

Several concepts were pursued, including the High Survivability Test Vehicle (Lightweight) and the Elevated Kinetic Energy (ELKE) gun system.

In 1983, the Naval Surface Weapons Research Center Laboratory mounted a 105 mm caliber gun onto a Sheridan chassis.

[55] From 1978 to 1980, under the joint Army–Marine Corps Advanced Antiarmor Vehicle Evaluation (ARMVAL), TACOM rebuilt 10 Sheridans with the General Motors 8V53T diesel engine and improved suspension.

In comparison, high-velocity anti-tank guns over-penetrated soft targets, while smaller caliber weapons could not carry as great a payload.

This was dangerous to the crew as the smoldering case could prematurely detonate the next loaded round, as happened three times during five months of testing from November 1966 to March 1967.

The expensive missile was fired in anger only in the Persian Gulf War's Operation Desert Storm, despite a production run of 88,000 units.

[76] This could be opened up into a sloping vertical surface in front of the driver, providing a bow of a boat hull, about level with the top of the turret.

Fabric formed the rest of the water barrier, folding up from compartments lining the upper corner where the side met the top of the hull, and held up at the back with poles.

The front of the "hull" was provided with a plastic window, but in practice it was found that water splashing onto it made it almost useless, and the driver instead usually had to take steering directions from the vehicle commander.

The Low Altitude Parachute Extraction System (LAPES) is a somewhat risky maneuver that allows accurate delivery onto a field when landing is not possible, and the practice was stopped in the late 1990s.