In the emmidiate post-WWII era the US Army introduced the M41 tank into service to fill their light tank role, armed with the war-era British designed 76mm gun. However the lifetime of this system was fairly short, the 25 ton tank was considered too heavy to be a true light tank, and had a rather short cruising range. Plans started to build an even lighter replacement mounting the same gun, resulting in the T-71 and T-92 test designs. The 19 tonne T-92 was later ordered to the tune of two prototypes.
However as the prototypes were entering testing, information about the new Soviet PT-76 tank became available. The PT-76 was amphibious, and soon there were demands that any US light tank be able to swim as well. The T-92 was too far into the design to be refitted, so the design of an entirely new system started as the XM551, no longer known as a "light tank", but instead "armored reconnaissance/airborne assault vehicle".
The need for even lighter weight presented the design with a particularily difficult problem; guns capable of defeating modern tanks at reasonable ranges were so large as to make the vehicles able to carry them far too large and heavy to be used in the light tank role. The use of HEAT rounds instead of conventional penetrating ammunition could address this, but HEAT rounds work better at larger calibres. Gun weight is typically a function of the calibre and muzzle velocity, so in the case of the XM551 they sacrificed the latter, producing the M81 152mm gun with very low muzzle velocity.
The M81 would allow the XM551 to deal with most tanks, but only at short ranges due to the low accuracy of the "lofted" low-velocity rounds. At longer ranges the tank would be vunerable, but it appeared there might be a solution to this problem. The solution was to equip the tank with gun-fired anti-tank missiles. 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. The XM551 appeared to offer the best of both worlds, for infantry support the large calibre gun allowed it to fire full-sized atillery rounds and cannister shot, while also giving it reasonable anti-tank performace 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 was based on an aluminum-armored multipurpose tracked vehicle, powered by a large 300hp diesel engine. The XM551 thus had an excellent power-to-weight ratio and mobility, able to run at speeds up to 45mph, which at that time was unheard of for a tracked vehicle. Unfortunately the the armor was thin enough that it could be penetrated even by heavy machine gun rounds, and is apparently particularily vunerable to mines.
Swimming capability was provided in a unique and somewhat odd fashion. The front armor was actually three folded layers, hinged together. They could be opened up into a sloping vertical surface in front of the driver providing a bow of a boat hull, about even with the top of the turret. Fabric formed the rest of the hull, folding up from hatches 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 make it basically useless and the driver instead had to stand up to see.
Production started in 1966, and reached service in 1968 as the Sheridan. 1,562 M551s were built between 1966 and 1970. The M81 gun had problems with cracks developing near the breach after repeated firing, a problem that was later tracked to the "key" on the missiles that ran in a slot cut into the barrel. Most field units were modified to help address the problem, but later the modified M81E1 was introduced with a shallower slot, along with a similar modification to the missile, that cured the problem.
The Sheridan saw limited action in Vietnam. Like the Ontos, the battle reports from the troops were glowing, while the reports higher up the chain of command were entirely negative. Of course an anti-tank vehicle being fielded against an enemy that didn't use tanks was something of a waste, but the gun proved an able anti-personelle weapon, and was generally loved by the infantry who were desperate for direct-fire support. In this role the real problem with the Sheridan was it's limited ammunition load, of only 20 rounds and 8 missiles. A common field-modification was to mount a large steel shield around the commander's 50cal gun, allowing it to be fired with some level of protection.
The Army started to phase out the Sheridan in 1978, although at the time there was no real replacement. Nevertheless the 82nd Airborne were able to keep them on, to the extent of 57 machines today, as it was the only air-deloyable tank in the inventory, and as an elite force they had considerably more "pull" than general infantry and armor units who were forced to get rid of them. Their units were upgraded to the M551A1 model, including a thermal sighting system for the commander and gunner. Sheridans were used Operation Just Cause in Panama in 1989, and were again lauded by their operators as providing firepower in needed situations. The Sheridan was also deployed in Operation Desert Shield in 1991.
Several attempts to upgun or replace the Sheridan have been made over the years since it was introduced, but none have yet been successful. Several experimental versions of the Sheridan mounting a new turret carrying the NATO-standard 105mm gun were made, but the recoil was so great as to make it almost unusable. Several vehicles were tested as a part of the Armored Gun System effort of the 1980s, but none of these entered service. Today the Stryker is intended to replace the Sheridan at long last, but many have commented that the Stryker's APC-based hull is even less capable than the Sheridan. Perhaps more annoyingly the 105 is a dedicated anti-armor gun and therefore less useful in the anti-personelle role than the M81.
The most ironic part of the M511 story is that it was created in order to give the US forces an amphibious tank, causing the existing T-92 to be adbandoned. However for all the trouble, it appears that the swimming system of the M511 was never used in combat.