Cold War, USA

The M103 – America’s Last Heavy Tank

After the Second World War the US military realized that some nations, particularly the Soviet Union, were outpacing them in tank development. In trying to reduce this gap, the US produced the M103; a huge, powerful and impressive-looking heavy tank that could knock out virtually any tank in service at the time.

But while it may have looked the part, the M103 proved to be the last breath of a dying breed – that breed being the heavy tank itself.

It originates from the T29, an equally mighty heavy tank developed at the end of the Second World War that didn’t make it into service. It would take another 12 years before the first M103s entered operational service, due to many factors.

Chief among these factors was the haphazard approach of American weapons research and development, along with interservice infighting on what should be the priority for upgrading its heavy tank class of vehicles.

M103 at a museum.
The M103 was one of the biggest and most powerful tanks of its era. Image by Skaarup.HA CC BY-SA 4.0.

Ultimately, the M103 would quickly fall out of favor with the Army in lieu of the M60 tank, but it was embraced with open arms by the Marine Corps. Eventually serving almost twenty years with the Corps, the M103 proved to be a linchpin in Cold War amphibious doctrine. However, due to doctrine and budget constraints, the Marines would have to overcome various engineering and tactical challenges in employing the M103 that would ultimately keep the program small.

The M103 would end its career as the last American heavy tank.


The T43

As discussed in a previous article about the T29, the US military recognized the need to upgrade its armor capabilities even before the end of World War Two. Chief among the desired upgrades was a proper heavy tank. The Soviets had already built the vaunted IS-3 tank starting in 1944. With its huge 122mm gun and 110mm of frontal armor, the tank was a force to be reckoned with.

IS-3 heavy tank in a museum.
The IS-3 shocked Western leaders when it was revealed. However it turned out to be rather lacklustre. Image by Billyhill CC BY-SA 4.0

While the British built the Conqueror to counter this threat, the US military had no such replacement. Due to this threat and with the unexpected beginning of the Korean War in 1950, the US Army was soon scrambling to begin production of a new heavy tank.

Because of the considerable advances in anti-tank technology over the past few years, infantry commanders wanted a tank that could not only take a wallop but had enough firepower to take out the strongest fortified positions soldiers could face.

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The US had already had some experience with a tank of this calibre thanks to the T29 and its variants, which just missed the Second World War but were used for testing in the years afterward.

T29 Heavy Tank at the Aberdeen Proving Ground
The T29, a late-WWII US tank that eventually evolved into the M103.

The T34 (a development of the T29 heavy tank) served as the general basis for the new heavy tank. Its hull was shortened and the frontal armor was rounded into a cast “beak”. Likewise, a heavily rounded turret was designed for the tank. It was designated T43.

The Marine Corps was more interested than the Army in a heavy tank. Because the Marine Corps did not have the budget for the research and development of a heavy tank, the Commandant of the Marine Corps personally wrote about the Corps’ need for a heavy tank to Army leadership after the conclusion of its own armor working group.

Several Marine officers who had served during the amphibious landing at Peleliu were leading the working group. Here, a Japanese tank counterattack had almost pushed Marines off the beachhead. The officers noted that the situation would have been critical had the Japanese been armed with heavier tanks.

An M26 Pershing at Fort Benning.
The M26 Pershing was the heaviest US tank fielded during the war. It is rated as an equal to the Tiger I. Image by Pierre-Olivier B CC BY-SA 2.0.

Because of this experience, these officers advocated for a heavy tank that could support the critical moments of an amphibious landing to protect the beachhead. For this reason, the Marines became the biggest advocates for the program and constantly needed the Army to continue development.

By 1955, Chrysler had completed the initial production run of 300 T43 tanks. However, the Army found them unfit for service due to around 144 different modifications that testing at Aberdeen Proving Grounds had identified over the past several years.

With some convincing by the Marine Corps, Army leadership approved the transfer of 74 T43s into service, now designated as the M103, to be troop tested by soldiers in West Germany. The remaining 220 tanks were to be upgraded with the desired modifications the Marine Corps wanted, and these would become the M103A1 models.

M103 heavy tank at The Tank Museum, Bovington.
An M103 at The Tank Museum, Bovington. This particular version is an M103A2. Image by 270862 CC BY-ND 2.0.

By 1955, the Army was not as invested in a heavy tank as it was ten years prior for two main reasons. Firstly, the much-feared IS-3 was found to not be as potent as Army intelligence once thought. Secondly, with a 144 required design changes, including some major work to the turret and fire control system, the Army adopted an attitude of making the best of the proverbial lemon they had bought from Chrysler.

Opting to make just 98 of the 144 recommended changes, the T43E1, once approved, became the M103. Because of this, the base model M103 would essentially remain the same in terms of dimensions and weight for the duration of its service, with modifications mainly affecting the fire control systems and engine.

The M103

The M103 had a crew of five personnel. There was a commander, a driver, a gunner, and two loaders. The two loaders were present because the M103 fired two-piece ammunition. Unlike ramming a standard charge, the charges for the M103 were self-enclosed in brass with a primer. Like a naval gun, the tank spits out the expended brass casing after each shot.

120 mm round for the M58 gun.
A Marine stood beside a round fired by the M103. This image clearly shows the scale of the M103’s ammunition.

Its gun was perhaps its most impressive feature. It was a 120 mm M58 rifled gun, which was 7.6 meters (25 ft) long and could fire projectiles at up to 1,150 meters per second. With the M358 armor piercing round it could penetrate 200 mm of armor angled at 30 degrees from 1,830 meters (6,000 ft) away.

In fact, the M58 is one of the most powerful rifled tank guns to have ever entered service.

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As for dimensions, the tank was around 11.4 meters (37 ft 5 in) long, 3.6 meters (11 ft 11 in) wide, and 3.5 meters (11 ft 8 in) tall. Weighing in at a combat load of 62.5 tons, the tank was the heaviest the US military had ever fielded at this point in time. A gasoline-fueled Continental AV-1790-7C engine was installed to propel all this weight. It produced 800 hp and 1600 ft-lbs of torque.

AV-1790 air cooled V12.
A Continental AV-1790 V12 engine. This example was from an M47 Patton, but the type was used in various forms in many US vehicles.

Because of this, the chief complaint among its Army users in Europe was that the vehicle was immensely underpowered. With a power-to-weight ratio of just 12.96 hp-per-ton, it is no wonder that the Army did not care for the tank much.

This was in part because of Army studies during World War Two that found most German heavy tanks were lost in the retreat instead of in an assault. Such was the case of being too heavily armored and too underpowered.

Further adding fuel to the fire was the inability of the tank to go long distances before major repairs. Field testing in West Germany showed that only after about 500 miles both the transmission and engine would need to be replaced due to wear and tear. It is probably because of this that the Army put the tank on overwatch duties, similarly to Britain with the Conqueror.

Despite its lack of mobility, it had strong armored protection for its crew. Sporting a glacis and turret with 130 mm (5 in) thick armor at 60 degrees, and a 250 mm (9.8 in) thick gun mantlet, there was little on the battlefield that could penetrate an M103 from a frontal assault. However, its side and top armor were noticeably weaker, reaching as low as 44 mm in areas (1.7 in).

Of note was its massive elliptical turret. By creating a sloped turret around all sides, the engineers could give the tank more protection while also cutting down on its weight.

M103 with turret turned.
The M103 had a huge turret, containing a centrally-positioned commander, a gunner, and two loaders.

Military leadership was likely fine with this since the tank was never intended to get up close and personal with enemy combatants. Rather, with its long range and heavy frontal armor, the Army envisioned the tank to stand off and take out targets from a distance. This was a problem for the Marine Corps though, as they desired more sophisticated fire control and sighting equipment.

The M103A1

While the Army figured that 98 of the changes from the T43 program were enough, the Marine Corps fought for all 144 changes to be implemented before accepting the vehicles into service. The Marines were adamant about this because these vehicles were the backbone of their armored support during an amphibious landing.

Unlike Army doctrine that called for M103s to stand off and provide overwatch, Marine Corps doctrine had M103s in the thick of it, supporting infantry during amphibious assaults.

An M103A1. The M103A1 can be quickly spotted by its low-profile engine deck and the bulge on the upper right side of the turret, behind the gun. This bulge accommodated the gunner and his optics.

With the likelihood of direct tank-on-tank combat much greater than for the Army, the Marines wanted to ensure their tankers stood a fighting chance. Chief among the measures the Marines required for their vehicles was the addition of advanced sighting and fire control technology.

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Unlike the M103, the M103A1 focused more on engaging in tank-on-tank combat. To accomplish this, the first significant change was adding several more periscopes and direct gun sights. The commander was also given an analog fire control computer not present on the M103.

M103 turret bustle heat deflector.
Petrol powered versions of the M103 had the exhaust mounted on top of the engine deck. This was troublesome, as the hot exhaust gasses would heat up the bottom of the turret below the crew. An exhaust deflector was fitted underneath the turret bustle, as seen here. Image by D. Miller CC BY 2.0.

The fire control computer was quite powerful for its time. It could receive direct inputs from the range finder and the type of ammunition being fired to create more accurate fire control solutions for gun lay and elevation.

The tank also received a huge upgrade in its gun mount technology. Before, the M103 sported a simple hydroelectric closed loop system that used one motor to adjust the gun. With the M103A1, engineers scrapped this system for a much more complex one with numerous servo valves and hydraulic motors.

M103 of the A2 variant.
The M103’s rangefinder armored covers can be seen towards the rear of the turret, on its upper side in front of the commander’s hatch. Image by Greg Goebel CC BY-SA 2.0.

The addition of these components made the turret and gun slew faster, resulting in the gunner making more fine-tuned corrections than previously possible with the M103 base model.

The M103A2

Starting in the early 1960s, the Army began to move away from the dated WWII doctrine of tank classes and transition to the concept of a single main battle tank. Their first ground-up attempt at this concept, the M60 tank, began to roll out in 1959. Though the Army had only been using the M103 for several years at this point, they began to phase them out of service. The Marines, however, were steadfast in their love of the M103A1.

Because the Marines did not consider the M60 a proper replacement for the M103, they agreed to upgrade the M103A1 as much as they could. Because of this, they opted to rip out the old gasoline Continental engines and replace them with the new Continental diesel AVDS-1790-2A engines – and corresponding transmissions – being fielded in the new M60 tanks.

M103A2 tank rear.
The M103A2 can easily be spotted by its raised engine deck and louvers at the rear. Image by Bachcell CC BY-SA 3.0.

The new engines did much to increase the combat power of the M103. Though horsepower remained relatively modest at 750 hp, the engine provided more torque – 1710 ft-lbs – and much more range. The less fuel-efficient gasoline engines had a combat range of just 80 miles. But with the new diesel engines, not only was the fuel capacity increased from 280 gallons to 440 gallons, the range increased to an astonishing 300 miles. The new engine also increased its top speed from 21 to 23 mph.

Service History

Army officials sent their 72 M103 tanks accepted into service to Hanau, West Germany, to become the 899th Heavy Tank Battalion. After several years of service, the battalion was redesignated as the 2nd Heavy Tank Battalion, 33rd Armored Regiment, to pay homage to its WWII lineage. The battalion also took up a rather odd way of incorporating the tanks into American positions.

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Divided into four companies of 18 tanks each, no officers above the platoon level had direct command tanks. Instead, the battalion sent its platoons all over West Germany to be attached to larger armored formations and never served as an independent unit. Of note, the Army obviously became dissatisfied with their M103 base models and requested 72 additional M103A1 tanks from the Marine Corps in 1959. The Marines allowed the loan, and for the rest of the time the Army operated them in Europe, they used the M103A1 models.

M103 heading to ship.
A M103A2 approaching the ramp of the U.S. Navy landing ship, in 1973.

As for the Marines, each of their three active tank battalions and its reserve tank battalion operated a company of M103A1 and then M103A2 tanks. These vehicles became a mainstay of Marine power projection and were frequent centerpieces in amphibious assault exercises and providing gunfire support in austere locations like Guantanamo Bay.

While the Marines fought back against the Army for over a decade, it was not until the Marine Corps officially accepted the M60 into Marine Corps service in 1972 that all active M103 companies stood down in 1974, when the M103 was retired from service in the reserve tank battalion. And despite its special place in Marines’ hearts, the M103, nor any of its variants, ever saw combat with either branch.

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The M103 may have never seen combat, nor was it a particularly good design, but it has the distinction of being the last official heavy tank fielded by the US. Its demise symbolised the transition of philosophies, from the brute-force methods of WWII and before, to the more streamlined, precision based methods of today.