Just like the British Conqueror tank, the M103 was designed to defeat Soviet Heavy tanks like the Josif stalin tank and the Just arrived T-10 during a conventional WW3.
The united states army did not used the Main battle tank concept first. Infact the concept truly appeared with the T62, M60, Leopard 1....
The US army in terms, reclassified there tanks via gun calibre, not neccesarly weight. Below 90mm became a light tank. The M24 chaffee and the new M41 walker bulldog. The Medium tanks where those off the standard 90mm gun. The M26 pershing and the M46-47-48 patton tanks. Above 90mm became the heavy tanks. In this case the M103.
The M103 shared many parts with the M47-48 patton to ease logistics. The engine was the same, and the hull was virtually the same next to enlarged and an extra roadwheel
The main armament was the 120mm M58 gun. Ammo capacity was 33 rounds. Penetration sources vary. But was over 280mm at 1000meters. The first version had faulty fire controls, but the M103A1 had the new sight (Steroscopic T52) and T33 ballistic computer. This gave the M103 imense accuracy and performance.(Just like the 105mm, the 120mm was so accurate it could easily hit captured german stahlhelms at ranges up to 1000 meters). Also usefull things was a semi-caseless shell. There was no empty shell casing once the tanks gun was fired, because the shell casing would burn up along with the explosion
M103 1957. 300 built.
M103A1 1959. 219 converted or rebuilt. New sight (Steroscopic T52) and T33 ballistic computer. Removed one coaxial machine gun. New turret electric amplidyne system traverse. Turret basket.
M103A2 1964. 153 converted or rebuilt. New 750 hp (559 kW) diesel engine from the M60 tank, increasing the road range to 480 km at the expense of a lower maximum speed of 37 km/h. New sight coincidence XM2A.
M103 Hull armour=
M103 Turret Armour=
A good source if you can read polish=