The M103 was designed to counter Soviet heavies such as the Josef Stalin tank or the T-10 if a conventional World War III broke out. Its long-ranged 120 mm cannon was designed to hit enemy tanks at extreme distances, but it was never used in combat. In 1953-1954 a series of 300 tanks, initially designated T43E1, were built by Chrysler at the Newark plant. Testing was unsatisfactory, and the tanks were all stored in August 1955. Only after recommending improvements, on 26 April 1956 the tank was standardized as the M103 Heavy Tank. Of the 300 T43E1s built, 80 went to the US Army (74 of which were rebuilt to M103 standard), and 220 were accepted by the US Marine Corps, to be used as infantry support, rebuilt to improved M103A1, then M103A2 standards.
In Europe, the US Army fielded only one battalion of heavy tanks, from January 1958, originally assigned to the 899th Armor, later re-designated the 2/33rd Armor. The US Army heavy armor battalion, in contrast to other armor units, was organized into four tank companies, composed of six platoons each, of which each platoon contained three M103's, for a total of 18 tanks per company.
The US Army deactivated its heavy armor units with the reception of the new M60 series main battle tanks in 1963.
T43E1 1953. 300 built.
M103 1957. 74 converted.
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 and maximum speed to 37 km/h. New sight coincidence XM2A.
U.S. 120MM Armor Piercing-Tracer (AP-T) projectile M358E1 for M58 gun pierced armor the old fashioned way - brute force from a big, tough projectile fired at high velocity, none of that fancy shaped charge or skinny little discarding sabot stuff. Muzzle velocity on these was an incredible 3,500 feet per second! This was a separate loading round which used the Propelling Charge, M46 using 28 pounds of powder in a 120mm Cartridge Case M109.
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).
This is the only ammo information I could find (found TONS of stuff on the powder charges tho).
Starting at pg 190
M103 Info
Tanks Net
American Military History
AFV Blog (1)
AFV Blog (2)