Instead of certain death, the T43 was progressed through an official order for 80 tanks requested by US Army to shore up its outdated stocks in December of 1950. The USMC sought a long-term replacement and engaged in a production order for 195 vehicles to which the total then grew to 220 requested units in 1951. In June of 1951, the initial T43 pilot vehicle was completed by Chrysler out of its Newark, Delaware facility and made ready for formal evaluations at the famous Aberdeen Proving Ground in Maryland.
The new design quickly showcased a myriad of technological and mechanical issues that required serious attention. Changes were requested that included increased armor protection which, in turn, increased operating weights and led to a reduction in speed. The main gun was updated to the 120mm T123E1 and more subtle changes greeted the internal fighting compartment. Testing was hurried which overlooked existing issues not entirely ironed out in an effort to have the tank ready for service in the Korean War. After passing the requisite Army tests, the vehicle was formally accepted for serial production under the "T43E1" designation.
The T123E1 main gun offered considerable fire power at range over that as seen by the upgunned M4 Shermans, the M26 Pershings and the M46 Pattons. It was designed to fire a 32lb High-Velocity, Armor Piercing (HVAP), a 23lb Armor-Piercing Discarding Sabot (APDS) and a 50lb Armor Piercing Capped (APC) projectile at a muzzle velocity of 3,300 to 4,200 feet per second depending on projectile in question. This allowed for penetration of 13.5 inch armor or less at ranges out to 1,000 yards. Of course the size of 120mm projectiles within the tank turret and hull limited total available projectiles carried. While mobility was lacking, the design certainly could give the Soviet offerings a challenge at the ranges expected in both Korea and Europe. Original M103 models were given 2 x 7.62mm coaxial machine guns while the later M103A1 introduced only a single fitting. A 12.7mm Browning M2 anti-aircraft gun could be fitted to the turret for close-in defense. The M103 housed 38 ready-to-fire 120mm projectiles, 5,250 rounds of 7.62mm ammunition and 1,000 rounds of 12.7mm ammunition.
Production of the T43E1 was underway in December of 1942 and would continue into June of 1954 by which point the war in Korea was over through an uneasy armistice. The T43E1 was adopted into the US military inventory as the "M103" with no nickname ever being assigned. Total production netted 300 examples from the Chrysler Newark plant.
The hasty arrival of the M103 necessitated additional modifications heading into 1956 with a pair of T43A2 pilot vehicles to serve as needed testbeds for evaluation. M103s were shipped to Europe to manage strategic chokepoints against an impending Soviet ground invasion. The US Army charged 74 for improved modification and these were then returned to active service. However, the USMC required even more in-depth changes to their M103s before it could be operationally introduced with computerized fire control and a revised turret. Based on the T43A2 prototype, the USMC eventually accepted 220 USMC M103s under the new designation of "M103A1" with deliveries occurring in January of 1959. 219 M103s were either converted or rebuilt to this new standard. US Army M103s arrived in Europe to the 899th Heavy Tank Battalion of the 7th Army, replacing their M48s then in service. Ironically, the US Army then requested, and was approved, to receive 72 M103A1 units from the USMC stock. These tanks joined their brethren in May of 1959 and maintained their positions until 1963 to which they were handed back to the USMC.
While the US Army had all but given up on the M103 by this point, the USMC soldiered on with the M103 as their heavy tank for the foreseeable future. The "MBT-70" joint tank program between the United States and Germany was to usher in the next generation of American fighting tank. However, prior to its arrival, there would have to be a period fulfilled with a low-cost interim solution. Instead of acquiring the US Army's latest Main Battle Tank - the M60 Patton - the USMC elected to installed M60 components to their existing M103 line. This included use of the same Continental AVDS-1790-2A diesel engines coupled to and all-new internal fuel stores. The modernization program began in August of 1963 with deliveries of the new systems in May of 1964. The M103A2 proved a noticeable upgrade to the M103 line as a whole with both operating speeds (23mph from 21mph) and operational ranges (300 miles from 80 miles) both increased. The M103A2 shared much of the same automotive components and rangefinding equipment as the M60 MBT and 153 examples were either rebuilt or converted for need.
The end of the line for the M103 came in 1972 when all existing units were retired from USMC service. The type was completely out of circulation by 1974. Left without a viable tank option (the MBT-70 program fell to naught and the M48 was highly outdated without the prospect for modernization), the USMC was forced to procure the US Army M60 MBT in number. The M60 replaced all existing USMC mounts from there on and remained the standard USMC main combat tank for the decades precding the arrival of the M1 Abrams.
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