Monday 29 August 2022

The Longest Millimeter

The first tanks of the Medium Tank M4 family saw combat for the first time in the summer of 1942, during the Second Battle for El Alamein. Combining excellent mobility, thick armour, and firepower that could defeat any enemy tank, the “General Sherman” was a solid contender for the title of the best tank on the continent upon its debut. However, progress did not stand still. The German army began fielding improved Pz.Kpfw.III and Pz.Kpfw.IV tanks, equipped with thicker armour and longer guns. Thankfully for the Sherman, American tank designers began to think about its modernization even before it reached the battlefield.

3 inches or 76 millimeters

The Sherman was far from the only armoured vehicle to make its debut in North Africa. The Gun Motor Carriage M10 went into battle alongside the Sherman, carrying the M7 gun with ballistics of the 3” M3 AA gun. The Sherman was conceived from the start to use a gun of this caliber, but it was not as simple as just moving the gun: the mass and size had to be reduced for installation in the turret of a medium tank.

The T12 3” AA gun was taken as the basis for the new tank gun. Here is where the indexes become jumbled. The new tank gun was called “75 mm T13”, which caused much confusion in correspondence. For clarity’s sake, the gun was renamed “76 mm T1”. The designation changed from the imperial system to metric for a very good reason. The length of the barrel was reduced from 57 calibers to 52. In order to preserve the ballistics of the gun, special ammunition with an increased powder load was used, which was incompatible with the old 3” gun. The 14.9 lbs (8.76 kg) M62 shell, fired from the gun at a velocity of 2600 fps (792.5 m/s) penetrated 112 mm of armour at 20 degrees at 500 yards (457 m), or 99 from 1000 yards (914 m). Aside from armour piercing ammunition, the gun fired M42A1 HE-fragmentation shells and M88 smoke shells. The shells were placed in the M26 casing. The new gun fit into the standard Combination Gun Mount M34 used by Sherman tanks armed with 75 mm guns.

M10 tank destroyer with a 3” gun, rear view. This gun had improved penetration compared to the 75 mm M3, but it was not so easy to put it into a medium tank

It seemed that the installation of the new gun into the Medium Tank M4 would be easy. The ammunition racks and gun mantlet were replaced, since the tankers wanted to have the 3x M51 (T60) telescopic sight in addition to the M47 1.5x periscopic sight. These sights allowed the gun to fire at a range of up to 3 kilometers. The new tank carried 83 shells for the gun, 2000 rounds of ammunition for its machine guns, and 600 rounds for the Thompson submachine gun. Two T1 guns arrived at the Aberdeen Proving Grounds by August 1st, 1942. One gun was installed in a tank with registration number W-3060572, the second was installed in a test rig and later tested in a tank with registration number W-3015305. Since the recoil length was only 12 inches (305 mm), the gun retained a good vertical aiming range: from −12.5 to +25 degrees in the improved Combination Gun Mount M34A1.

Medium Tank M4 with a 76 mm T1 gun in the Combination Gun Mount M34. Aberdeen Proving Grounds, August 1942. The white ring shows the length of the 75 mm M3 gun

On August 17th, 1942, the Armor Board recommended that the tank be standardized under the index M4A1(76 M1). A series of 1000 tanks was proposed.

M4A1(76 M1) on trials at Fort Knox. February 1943

The Pressed Steel Car Company began assembly of a pilot batch of Medium Tanks M4A1(76 M1) in early 1941. Two tanks arrived at Fort Knox for trials in February. This time things did not go as smoothly, as the testers uncovered one issue after another. No changes were made to the turret in order to improve the operation of the larger gun, and only a 363 kg counterweight was placed in the bustle to help with balance. The testers evaluated the position of the gunner, his seat, and his instruments, as unsatisfactory. The elevation flywheel was judged as “spongy and sluggish”, which made precise aiming impossible. The tank also lacked a “telescopic sight with sufficient magnification to fully realize the potential of the 76 mm gun”. The 310 kg counterweight on the recoil guard was also found unsatisfactory. In order to evacuate the tank, the loader had to first fold it out of the way, a difficult task even without the stress of battle. The testers recommended that either the railing needs to be lightened, or the loader needs his own hatch in the roof of the turret. The crewmen in the turret were not the only ones who suffered from the new gun: the travel clamp blocked the driver’s vision. In addition, it was impossible to disengage it without exiting the tank. The trials report ended with a long list of design changes required to make working with the new gun acceptable. Naturally, such a sobering evaluation had an effect on the Armor Board. On April 5th, 1943, the recommendation for standardization was recalled. The Aberdeen Proving Grounds, Armor Board, and Tank Destroyer Board each received one M4A1(76 M1) tank. The rest were converted into regular 75 mm gun tanks.

T20 intermission

The military didn’t stand still while the drama around the 76 mm gun developed. In August of 1942, requirements for a new tank were composed at a conference in Fort Knox. The tank had to weigh no more than 30 tons, and have three interchangeable turrets: one with a 75 mm gun equipped with an automatic loading mechanism, one with the 76 mm M1 gun, and one with the M7 3” gun. American engineers were also to test out new types of transmissions. Six new tanks were proposed. Two tanks, indexed T20, would be built with a new transmission. Two more tanks would be equipped with a modified version of the Sherman’s transmission. These tanks were indexed T22. The two remaining tanks were indexed T23, and would use an electric transmission like the one designed by General Electric for the Heavy Tank T1E1.

The Medium Tanks T20, T22, and T23 would carry the 76 mm M1 gun. Tanks with the E1 suffix would have the old 75 mm gun, and the E2 suffix was reserved for variants with the M7 gun. Most of the tanks from this family were never built, since the 76 mm gun proved itself more than satisfactory. The only exception was the T22E1, which received a 75 mm gun. The other E1 and E2 variants were never built.

A Medium Tank M4A1(76)W and a GMC M10 outside of the Overlord Museum in Normandy. Both have 76 mm guns with similar ballistics, but their ammunition is incompatible

The program was very promising. In August of 1942, the Americans let the Canadians know that a new tank would enter production in 1943, and it would be preferable to manufacture the same tank in both countries, instead of the Medium Tank M4 in the US and Ram II in Canada. However. the due dates slowly slipped. By July of 1943, the Canadians were informed that the new tank would only be available in early 1944. There was some cause for worry, as the Americans admitted that there were no concrete plans regarding production of any T20 series tanks, aside from prototypes. The Americans were upfront about the fact that the T20 needed a lot of work and would not enter production in its current state.

​Diagram of the turret roof of T20 series tanks. Due to the identical turret ring diameter, the possibility of using new turrets on old hulls was considered even in the early stages of the project

The Americans advertised their new tank without reservation. The British were told that the T20 would be so superior to any variant of the Sherman or Cromwell, that no army would want to go into battle in one of those tanks knowing that the T20 was available. Foreseeing massive demand for the T20, the Canadians estimated that these tanks would not be available to them until late 1944 or early 1945 at the earliest. This kind of production bottleneck seriously worried the British, who even viewed the Assault Tank T14 as a safer alternative.

The caution of the British proved correct. The T20 family was plagued by misfortune. The T20 turned out to be 2.5 tons heavier than calculated, and its transmission and suspension broke down constantly during trials. In addition, the tank overheated easily. The T20E3, a variant with a torsion bar suspension, burned up during trials. The T22 was removed from trials after an engine failure.

The T23 family was luckier. These tanks received new turrets, designed after the failure with the Medium Tank M4A1(76 M1). The first prototype carried the Combination Gun Mount T79 in a cast turret, designed on the basis of the standard Medium Tank M4 turret. The second turret was welded, with a gun mantlet that covered the entire front, akin to the German Pz.Kpfw.III. The 76 mm gun in this turret was installed in the Combination Gun Mount T80. Both turrets had identical turret baskets, which held 42 rounds of ammunition in ready racks. The second turret performed better in trials, and was eventually also made cast.

The T23 looked promising. An order for 250 tanks was made in May of 1943. To free up resources for Medium Tanks T25E1 and T26E1, the number was decreased to 200, but then increased back to 250 again. The first production Medium Tank T23 was assembled in October of 1943. Its improved turret carried the same Combination Gun Mount T80, but now had a commander’s cupola with an AA machinegun mount, a smoke grenade launcher, and a 3” thick gun mantlet. The tank was sent to Fort Knox, where the final verdict was made: the tank was unsuitable for service. The electric transmission was hard to work with, and all technicians in the army had to be retrained to repair it, and the entire stock of spare parts had to be refreshed. In a time of war, this was unacceptable. The Medium Tank T23 shared the fate of its ancestors, and never made it to the front lines. The Medium Tank T23E3 with a torsion bar suspension was also rejected.

Turret upgrade

The development of the T23 didn’t turn out to be pointless. The Americans decided to develop a backup plan back in the spring of 1943. T23 76 mm turrets would be made compatible with M4 tank hulls, since they had the same turret ring diameter. A T23 turret was installed on a hybrid M4 tank instead of the stock turret. This variant was indexed Medium Tank M4E6. Other improvements were made to this tank, most notably installation of wet ammunition racks, which reduced its vulnerability to devastating ammunition fires. The tank entered trials at the Aberdeen Proving Grounds in late June of 1943.

The gun also changed since the trials held in 1942. The recoil surface was lengthened by 305 mm, which allowed the gun to be moved forward and balanced better. The improved version of the gun was called M1A1.

Medium Tank M4E6

There were a lot of changes in the turret compared to the previous attempt at installation of a 76 mm gun. Issues with the elevation mechanism were solved using a split pinion gear with a spring. The stabilizer was also modified to deal with the heavier gun. The coaxial machinegun was moved two inches forward. The diameter of the turret basket increased, and an opening was cut in the floor to allow the loader to reach ammunition stored underneath. He also finally received a hatch in the turret roof. The amount of ammunition carried on board was reduced to 63 rounds.

The tank’s mass grew to 69,000 lbs (31,300 kg). The Continental R-975-C1, already the least reliable of all Sherman engines, struggled with the extra weight. A decision was made to change the gear ratios. The top speed in fifth gear dropped from 24 mph (36.6 kph) to 19.6 mph (31.5 kph). The Armor Board was categorically opposed to this solution, arguing that the tank must be capable of keeping up with its lighter comrades in battle. Trials showed that there was no reason to worry. A production M4A1 could cover a 1.5 mile (2.4 km) uphill stretch of road in 5:32, while the M4E6 managed to do it in 14 seconds less. Off-road, the situation was reversed: the M4A1 covered 5.6 miles (9 km) in 45:31.5, while the M4E6 needed 46:45. This meant that the tank lacked engine power. Testers demanded that the gear ratio be dropped further.

The new turret was criticized by the Medical Research Laboratory. The telescopic sight was still hard to use, since it was too far forward. Significant looseness in the sight mount meant that it was hard to aim precisely. The M9 rangefinder didn’t allow accurate shooting at a range of 2000 yards (1829 m). The crew’s seats were also far from perfect: the gunner’s seat was deemed “large and unwieldy”, and the commander’s seat could not be adjusted. The trigger mechanism pedals were poorly positioned. The crew’s actions were constrained by equipment. The loader’s hatch was judged too small. Locks for the turret hatches needed work.

Despite the aforementioned issues, the trials were deemed a success. The new design was standardized as the Combination Gun Mount M62.

​Top: experimental muzzle brakes for the M1A2 gun: T8, T8E1, T8E2. Bottom: standardized muzzle brake M2

This was not the end of 76 mm gun development. The 76 mm Gun T3 with a muzzle brake was designed to replace the M1A1. Trials showed that the idea was correct, but the T8 muzzle brake copied from German designs was unsatisfactory. The muzzle brake was improved by removing 3 pounds (1.4 kg) of excess weight, resulting in the variant called T8E1. The cast T8E1 was compared against the forged steel T8E2 and forged aluminium T8E3. Trials showed that the T8E3 was the least satisfactory option, while the characteristics of the T8E1 and T8E2 were equal. Since the T8E1 was easier to produce, it was standardized with the index M2, and the gun was standardized as the M1A2. The gun had a different rifling angle than its predecessor, with one twist in 32 calibers, instead of 40 calibers in the M1A1. Interestingly enough, guns began to receive threads for a muzzle brake before the design was finalized. As a result, there are many photographs of Sherman tanks with bare threading at the end of the barrel, or a counterweight, which also protected the thread. The M1A2 gun required an additional counterweight on the breech to balance out the 30 kg muzzle brake.

Old M1A1 gun barrels were also threaded to accept muzzle brakes. This variant was called M1A1C.

Top: experimental forged aluminium T8E3 muzzle brake. Bottom: Experimental cast T8E1 muzzle brake

As of August 17th, 1943, the American army wished to cease production of 75 mm gun tanks by January 15th, 1944, and switch to producing only 76 mm guns. The armored force requested 1001 tanks with a new gun as soon as possible for trials on the real battlefield. Even with the USA’s industrial might, the replacement of thousands of 75 mm gun tanks was a difficult issue. In addition, the Armor Board was against complete replacement of 75 mm gun tanks.

Comparison of the fragmentation effect of 75 mm HE rounds (left) and 76 mm HE rounds (right)

The 75 mm gun had a much more powerful HE-fragmentation round. Despite the smaller caliber, the hull of the shell was 567 g heavier, and contained an extra 312 g of explosives. This meant that the 75 mm M48 shell generated more than twice as many lethal fragments as the 76 mm M42A1 shell. The amount of fragments that could penetrate a 9.5 mm steel plate was incomparable: 78 for the M48, 5 for the M42A1. Against unprotected targets, the M48 covered a 33% larger area with lethal fragments than the M42A1.

The penetration of 76 mm AP shells compensated for the reduced effectiveness of the HE shell. By American standards, the gun could penetrate 108.2 mm of armour at point blank range at an angle of 30 degrees, compared to 79.5 mm for the 75 mm gun in the same conditions. This difference only grew with distance. Thanks to a higher muzzle velocity, the concrete penetration was higher for the 76 mm gun, even with the same unsatisfactory HE shell.

A penetration diagram for the Tiger tank. The 75 mm gun could not penetrate the front of the tank from any range, unlike the 76 mm

The plans for full rearmament had to be scaled back. A meeting on September 20th, 1943, established that it was best to fight with a mix of 75, 76, and 105 mm armed tanks. By the end of October, the proportion of 76 mm gun tanks requested was reduced to 25%.

Production of tanks with 76 mm guns began in January of 1944. The first 100 tanks armed with 76 mm guns were M4A1s, produced by the Pressed Steel Car Company. These first tanks were armed with M1A1 guns, and later M1A2. There were also tanks produced with the M1A1C. Production of M4A3 tanks with 76 mm guns began in March at Chrysler. In May of 1944, the first 30 M4A2(76)W tanks were assembled at the Detroit Tank Arsenal. Production of the M4(76)W was planned, but never started, the M4A4 was never even planned to receive a 76 mm gun.

Lend Lease evaluation

In the British Commonwealth armies, tanks with 76 mm guns received the suffix “a”. For example, the tank with W.D. number T.262414, which arrived at the Experimental Wing, was called Sherman IIa. By this time, the British already had their own tank with a more powerful 76 mm gun, the Sherman Firefly. Nevertheless, the British decided to test the American tank.

A Sherman IIA in the Vehicle Conservation Center at the Bovington Tank Museum

The lack of a muzzle brake disappointed the British. It appears that they received an early production tank with the M1A1 gun. The gun depressed to −10 degrees, and did not clip the hull while the turret turned. The maximum elevation of the gun was 25 degrees. According to the British, the gun was poorly balanced. The balance could be improved if the counterweight on the gun guard rail was removed, but then the stabilizer ceased to function. The guard rail also performed poorly: the welding seams began to crack after a few shots, and the shell casings bounced off the guard rail and hit the loader instead of deflecting into the bag. The bag could hold up to 30 casings, but the stabilizer wouldn’t work if 3 or more were present. The gun also climbed more and more with every shot as the number of casings in the bag increased. The coaxial Browning machinegun also had a bag for spent casings, which held 320 of them. The gun arrived with the M70H telescopic sight, but, since the opening in the gun mantlet was made for the much larger M71D sight, there was a lot of empty space around it.

Results of firing at a Sherman’s upper front cast armour with 75 and 76 mm shells. Only 76 mm shells were able to penetrate fully

The British mostly liked the aiming mechanisms. They didn’t like that the vertical aiming mechanism was too weak, and that the gun had to be locked in place during travel. Thankfully, the travel lock could easily be disengaged by the driver or his assistant.

The turret traverse mechanism was a pleasant surprise for the British. The Americans surpassed all expectations. The Oilgear power traverse was sensitive enough to follow a target moving at 5 mph (9.6 kph) at 2000 yards (1829 m). Due to the unbalanced turret it was impossible to traverse it by hand at a tilt of 15 degrees, but the powered traverse could still turn it, although not quite as well.

Oilgear powered traverse

The trigger mechanism had a backup: the gunner could fire either with a mechanical pedal or a button. The button was not on the aiming flywheel, which the British did not like, as he had to remove his hand from the aiming mechanism to fire.

The bulky turret traverse mechanism also had a severe drawback: it made the gunner’s station very cramped. The gunner had nowhere to put his right knee. If he twisted his body to position it comfortably, it was inconvenient to use the telescopic sight. British pickiness was not at fault here; the Americans discovered a similar flaw in their own trials.

The commander’s station also drew criticism. The seat was soft, but wobbled significantly, causing the commander to slip off in motion. The upper position of the seat was criticised for exposing the commander down to the waist, and not giving him anywhere to put his feet, but the lower position was deemed satisfactory. The new commander’s cupola was well received.

The loader’s station was disliked by British testers. Several improvements could be made. Moving the seat 7 inches (18 cm) forward could have eased the loader’s work. Modifications of the guard rail prevented spent shell casings from bouncing into the loader, improving the effective rate of fire. 5 rounds could be loaded from the ready rack in 30 seconds. If the assistant driver handed the loader ammo from below, then 5 rounds could be loaded in 15 seconds. In total, the tank contained 71 rounds of ammunition, 41 of which were available with the gun turned forward.

The assistant driver’s position did not draw any criticism, but it’s likely that it was not used much. This crewman was absent in the Sherman Firefly. Perhaps the British planned on removing him here as well, as the report comments that it was easier to access one of the ammunition racks if his seat was folded down.

​Canadian Shermans on training exercises alongside more modern Centurion tanks

The Sherman IIa had some good features, but it did not receive a lot of interest from the British tankers. In addition to all the drawbacks discovered during trials, the penetration of the 76 mm gun was lower than that of the British 17-pounder, and ammunition for the gun was not produced in the UK. However, the British were in no position to argue. As a result, the Sherman IIa did serve in the British army, but only in Italy, and even then in small amounts. This did not mean that the rest of the Commonwealth shared the distaste for the 76 mm gun. For example, the Canadian army replaced its Ram II tanks with the Medium Tank M4A2(76)W HVSS after the war and used the M4A3(76)W HVSS in the Korean war. 

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