Monday 27 February 2023

Modernization in the British Style

Great Britain, the nation that was first to invent the tank, lost its first place in tank building by the end of WWII. Nevertheless, the British designed the 17-pounder, a first class tank gun, and put it to good use on a number of vehicles, both domestic and imported ones. The most famous such vehicle was the Sherman Firefly.

Chassis for a big gun

The main British tank gun at the start of WWII was the 40 mm 2-pounder. This gun was enough against German light and medium tanks at first, but enemy tanks encountered in North Africa already had thicker armour. At first, extra protection came from applique armour that would fall off after 1-2 hits, but soon tanks with 50 mm of monolithic armour appeared that could only be penetrated at point-blank range.

17-pounder gun, The 17-pounder anti-tank gun was a powerful weapon, but vulnerable on the battlefield due to its size and weight. The muzzle brake on this gun is not original.

The need for more powerful tank guns was discussed as early as the summer of 1941. Arrivals of Lee and Grant tanks with the 75 mm M2 gun helped, but not for long. The American gun was deemed to be an acceptable interim measure until the arrival of sufficient quantities of towed 57 mm 6-pounder and 76 mm 17-pounder guns. The 6-pounder was small enough to fit into a tank turret, but the 17-pounder was far too large.

Meanwhile, the enemy wasn’t kind enough to wait. The armour of German tanks kept getting thicker, and soon the Tiger tank hit the battlefield. Preliminary inspection showed that it was most vulnerable to high velocity anti-tank guns in the 3” class. The Americans already had a motorized weapon like this, the 76 mm gun on the GMC M10, but the British 76 mm gun was only available as a towed anti-tank gun. These guns were effective, but also vulnerable. A report from the Imperial General Staff dated June 2nd, 1943, described a battle where six German tanks (two of which were Tigers) attacked a 17-pounder gun. The gun managed to knock out all six, but the last tank fired at the same time and destroyed the gun.

It was difficult to build a turret large enough to house this powerful gun, and British engineers came up with an easier way to add mobility to the gun. Tow hooks from 3-ton trucks were welded onto the back of tanks, turning them into ersatz artillery tractors. Of course, this solution was far from ideal. Unlike an artillery tractor, there was nowhere for the crew and ammunition to go. Artillery crews had to sit on the engine deck holding crates of ammo, which severely limited the amount of rounds they could carry.

The large engine access hatch in the rear of Sherman tanks prevented the tow hook from being mounted at the proper height.

There were plenty of other issues with this solution. The truck tow hook was not suitable for tanks. The hook was placed too low to the ground and could be damaged if the tank drove over an obstacle. It was not possible to place it any higher, as it would interfere with the engine access hatch. There were issues with tanks that didn’t have a hatch there too, as the 17-pounder gun’s long barrel would dip when driving on uneven terrain and could hit the ground. An artillery limber from the Ordnance QF 25-pounder gun could be used, but it was not a short term improvement as it would have to be converted to accommodate the longer 17-pounder rounds. Nevertheless, the Department of Tank Design approved this tow hook and ordered 5000 sets.

A better solution was developed in parallel. On June 8th, 1942, the Canadian Department of Munitions & Supply received a response to their inquiry on the use of the 17-pounder gun in tanks. According to this letter, a tank mount for the 17-pounder would be ready in two months. On June 9th the Canadian Chief of Staff, General Andrew McNaughton, was informed that in the future the Canadian Sherman tank might be ordered with a 17-pounder gun instead of a 75 mm gun. On July 25th this tank even received a name: the M4A1 armed with a 17-pounder would be called Buffalo II and the M4A2 would be designated Bear II. The Buffalo tank project eventually turned into the Grizzly tank, while the Bear was cancelled altogether.

A Canadian Grizzly tank (copy of the M4A1 Sherman) with one of the few Firefly turrets sent to Canada for training. Firefly tanks meant for combat were not built on the M4A1 chassis either in Canada or elsewhere.

The British planned to equip 30% of their tank fleet with 17-pounder guns, 60% with regular 75 mm guns, and 10% with 95 mm howitzers for close support. There were few candidates for compatible tanks in early 1943: the TOG heavy tank and the A30 Cruiser, later named Challenger. Work on the former stalled by this time, it was clear that the heavy and clumsy tank was unsuitable for the modern battlefield. The latter seemed more promising, but by the fall of 1943 it was clear that this was not the best option either.

GMC M10 tank destroyers began arriving in the UK by that point. These were suitable for installation of a more powerful gun than the 3” M7, and so the British decided to install a 17-pounder gun. A short while later a decision was made to do the same thing with the M4A4 tank, known in the British army as the Sherman V. Development began in the fall of 1943 and took place around the same time as the work on the M10, which caused some confusion.

An international tank

Blueprints were nearly ready by December 3rd, 1943, except the ammunition racks, which were only completed towards the end of the month. Tanks were being tested in parallel to determine if they were suitable for conversion. The British were afraid to overload the turrets, and so they were put through difficult trials indeed. Two 15 cwt (roughly 760 kg) weights were attached to the front and rear of the turret. Two M4A4 tanks were put through trials, one with an Oilgear turret traverse, another with the Westinghouse. Each tank travelled for over 1000 miles on and off road without the use of a travel lock. The turret was periodically rotated during movement. Both turret traverse mechanisms broke, but inspection showed that this was not due to a design flaw. The Oilgear traverse broke due to a manufacturing defect, the Westinghouse one was not assembled properly. It was decided that both mechanisms are suitable for use with 17-pounder guns, especially since the real Firefly turret gained less than a ton of weight. This decision was later revised and only tanks with Oilgear traverse were selected for conversion.

Sherman V and Vc tanks from the 5th Armoured Division. Aside from the longer gun, the Sherman Vc can be distinguished from its predecessor by an extension in the rear of the turret that houses the radio.

A new cradle, gun shield, and gun mantlet were designed for the new gun. The heavier gun also required a new elevation mechanism. A No.52 3x magnification telescopic gun sight was used. Two recoil brakes from the 6-pounder gun were installed, but this was still not enough to limit the recoil to a reasonable length, and there was not enough room to install a radio behind the gun. The British found a clever solution: the rear wall of the turret was cut off and a new bustle was attached that held the radio. The bustle extension doubled as a counterweight. The sides were 2” (51 mm) thick, the roof and floor 1” (25 mm) thick, and the rear was 2.5” (64 mm). This was not the end to the changes. Three exhaust fans were added to the roof of the turret, as well as a dedicated hatch for the loader. A mounting for the No.12 rangefinder was welded onto the commander’s cupola.

A large extension to the turret bustle was welded to the back of the turret. This extension housed the radio and doubled as a counterweight to balance the longer gun.

The tank carried 78 rounds of ammunition, 59 of which were stored under the turret basket: 40 in a bin to the right side and 19 to the left. A portion of the basket floor was cut out to allow access to the ammunition bins. The ready racks held 5 rounds of ammunition: three under the elevation mechanism and two in front of the loader. The assistant driver and bow machine gun were removed from the hull and 14 more rounds were stored in their place. Every ammunition bin above the turret basket floor was armoured to reduce the odds of detonation in case the tank was penetrated.

The tank also carried 20 boxes of ammunition for the .30 caliber machine guns, 6 boxes for the .50 cal, and 27 rounds for the 2” bomb thrower. The final design carried only 77 rounds of main gun ammunition, as one of the rounds held in the left rack under the floor was removed. It was difficult enough to load the tank with ammunition as it was. It took a well trained crew 18-22 minutes to refill their tank.

 Placement of ammunition in a Sherman Firefly tank

Gunnery trials took place on December 30th, 1943. The tank fired 100 shots with the turret forward, backward, and 90 degrees to the left and right in order to determine the stability of the tank. The engine was off and the tank had all hatches open. The crew was not inside when the gun was fired. The first 96 shots took place without any issues. The tank was a stable platform even for such a powerful gun and oscillations caused by firing the gun quickly died down. The only defect observed was failure to return to battery after firing 42 shots in 30 minutes. Secondary ignition of gases in the barrel was observed, but no flashback.

Everything changed when the hatches were closed for the 97th shot. Even with the engine idling at 1500 RPM enough fumes collected in the turret for flashback to occur. The flash could be seen through a 6-inch observation port cut into the turret bustle. Flashback was also seen after the 98th and 100th shots. Fumes in ejected casings that landed on the floor continued to burn after the flash in the turret subsided. A pause taken before the 99th shot allowed barrel to cool and the fumes did not ignite this time.

The flashback was not something unexpected, as it was seen before during trials of the Challenger, but this was still a serious drawback. Until a more permanent solution could be found the problem could be remedied by disengaging the semiautomatic mechanism and instituting a strict limit on the rate of fire. Overall, the installation of the new gun was deemed a success. No deformation of components was found and the turret could easily be traversed by hand after firing.

A Firefly from the 4th Armoured Brigade. A stowage box is welded to the front. This is commonly seen on this type of tank.

Further trials in January-February of 1944 revealed more defects. The semi-auto mechanism had issues, closing the breech on its own after extraction. The telescopic sight was unusable during rain, and the periscopes could only be used if the commander climbed out of the turret and wiped them down frequently. The safety mechanism broke during one of the trials, locking both the electric and manual firing mechanisms. The gun had to be fired by striking the breech with a mallet. Issues with burning fumes did not disappear. All these factors limited the rate of fire, which was not the best even in an ideal situation. The loader could keep up a rate of 5-6 RPM, after which it took 1.5-2 minutes to refill the ready racks. This was not considered a serious drawback, as the tank would likely be changing positions at the time. The British had no illusions about prolonged firing from one place, as the flash from the muzzle brake would give the tank’s position away. The muzzle brake was considered too heavy and made elevating the gun tough.

The 17-pounder gun had a distinctive bulbous muzzle brake.

Further trials showed that flashback was nearly nonexistent when firing HE shells. The flashback was also nearly harmless for the crew, only singeing the hairs on the loader’s hand if he didn’t retract it fast enough. Various solutions such as clearing the barrel with compressed air after firing were tried, but in the end the flashback was considered mild enough to ignore it, only a screen was installed to prevent it from distracting the commander. Trials of tanks with an additional extractor fan continued until March 20th. This addition was deemed to have solved the issue.

Reliability trials were also held in March. Special attention was paid to the behaviour of the ammunition racks. Testers reported on issues on March 23rd, after 941 km of driving on roads and 599 km off-road. It turned out that it was impossible to adjust the clutch and brakes as the mechanisms were blocked by ammunition racks. The racks also needed improvement, as ammunition stored in them jumped around during driving and could be damaged.

Defects encountered during 2000 mile trials. Defects from the first half of the trials are cross-hatched, defects in the second half are single hatched. The greatest amount of trouble was encountered with the running gear and suspension, especially in the second half.

Final trials of production tanks began in April. Weighing of a Firefly with the WD number T.2281715 showed that the mass of the rearmed tank reached 34.75 tons. The bogie tires, a known weak spot of M4 series tanks, suffered the most from this extra weight. It was decided that the marching speed would be limited to 15 mph (24 kph). Openings were cut in the ammunition bins to make adjustment of the clutch and brakes easier. The ammunition bins were equipped with sprung latches and the felt lining was replaced with leather, which made it easier to take out rounds. It turned out that the turret traverse mechanism was overloaded, but an improved version was already in the works. Gunnery trials showed that the concentration of CO fumes in the fighting compartment with the engine running did not exceed the maximum limit of 0.05%.

The tank drove for 2761 km without the use of a travel lock to test the reliability of the turret traverse. The mechanism passed these trials. The turret could be rotated even at a tilt of 15 degrees. Inspections after trials showed that the turret ring was packed with dirt after driving. This did not impede the work of the traverse mechanism, but could lead to corrosion over time.

Graph of major defects and minor defects against mileage. The number of minor defects shot up significantly after 1000 miles (1600 km) of driving.

There were also drawbacks to the radio bustle extension. The turret had to be turned backwards during travel due to the long gun, and the bustle extended above the driver’s head. This introduced a chance of hitting his head if the tank was driving on a bumpy road with the driver in the upper position. Even the improved ammunition racks continued to damage rounds. Despite these issues, the trials were deemed a success. The tank didn’t exhibit any previously unseen problems and the reliability didn’t differ much from that of an ordinary Sherman V.

Army trials did not go as smoothly. Compared to the Sherman V the rate of replacement of bogie tires went up by 120% and suspension springs by 440%. The military’s calculations showed that at 34.75 tons the suspension and final drives were working at their limit.

The tank was tested in comparative gunnery trials against its most likely opponent, the 7.5 cm Pak 40 anti-tank gun. It turned out that the German “flashless” powder still produced a flash, just not as much as the 17-pounder. The 17-pounder also produced a lot of brown-yellow smoke during firing, which was noticeable against the sky, but not against terrain. The white smoke produced by the Pak 40 was much more noticeable.

Sherman Ic with the name Rycerz I (Knight I). Sherman Ic tanks with hybrid hulls can be seen in the background.

The stream of Sherman V tanks suitable for conversion began to dry out in February of 1944. Production of this tank in the US ended. While the military searched for compatible tanks in armoured units, the FVPE (Fighting Vehicle Proving Establishment) tested several other chassis for conversion. Sherman I (M4) and Sherman II (M4A1) tanks weighed down with ballast were put through trials. The Sherman II showed 38 defects, 11 of which were major, during the 2000 mile run. Further trials showed that necessary components could not fit into the rounded hull. The British kept trying for several months, but a decision was made to stop trying to convert the Sherman II on April 14th, 1944.

There were a few Sherman IIc tanks built nevertheless. Canada ordered 3 turrets with 17-pounder guns to build Firefly tanks on the chassis of the Grizzly, a variant of the M4A1 produced in Canada. Since there was a deficit of turrets, the Canadians were sent training mounts to be installed in Canadian tanks. At least one tank was built this way. It’s hard to tell if the British ever built a Sherman IIc of their own. A list of Sherman variants used in the British army composed on June 21st, 1944, mentions a Sherman IIc. This tank is listed as having two Browning .30 caliber machine guns, whereas the Grizzly with a 17-pounder had its bow machine gun welded over like all Fireflies.

Defects exhibited by the Sherman I (left) and Sherman II (right). The Sherman I handled the additional weight much better.

Unlike the Sherman II, the Sherman I only exhibited 22 defects, 5 of which were major. The welded hull was suitable for conversion, although it could only fit 75 rounds of ammunition. There was an additional limitation. The front 20-round bin could not be filled with APCBC ammunition. The clutch adjustment mechanism protruded through a hole in this bin and there was not enough room to fit a ballistic cap. Only shorter rounds could be placed in one of the slots.

The bin under the left side of the turret basket was replaced with two bins fitting 8 rounds each. This somewhat simplified the loading process, but not a lot. The ready racks could be refilled in a minute with the gun turned right by 40 degrees or a minute and a half if the gun was pointed forward. If the position of the turret was less favourable, the process could take up to three minutes. The turret had to line up with the bins very precisely, which added extra time to the reloading process.

Ammunition storage in the Sherman Ic. As the diagram shows, most of it was stored underneath the turret basket floor.

The cramped fighting compartment also required the spent casing bag to be emptied, otherwise it was impossible to refill the ready racks. The loader had to throw out the casings from the top hatch, as the Americans started welding the pistol ports of Sherman tanks shut. The practice was stopped so that the pistol port could be used to throw out spent casings, but many loaders still had to put in the extra work to throw them out the top. Some testers even recommended removing the 14-round rack and returning the assistant driver so he could help pass ammunition up to the loader.

The layer of paint inside the tank was so thick that the ammunition bin lids couldn’t close after they were filled. The rearranging of the fighting compartment also forced the British to forego any lighting aside from a single lamp in the turret bustle. It was not bright enough to illuminate anything in the front of the turret, and the crew’s eyes had to get used to the darkness. A look into the periscopes would make the crewmen unable to see inside their own tank for several seconds.

Machine gun ammunition was stored in the sponsons and was easier to access.

Unlike previous trials, this tank was tested with the engine running. It turned out that after three shots the conditions in the fighting compartment became intolerable, as there was no exhaust fan in the turret. In normal conditions the rate of fire was the same as on the Sherman Vc: 5 RPM. There was another defect: the muzzle brake unscrewed itself during firing.

Like on the Sherman Vc, the turret was well balanced. It took 17 seconds to traverse it in a full circle using the electric traverse or 1.5 minutes by hand.

The Americans showed an interest in this new weapon, even discussing replacing the Pershing’s gun with it. There were several variants of this replacement. The first and simplest involved just replacing the gun, like on the M10. This tank couldn’t carry 70 rounds of ammunition as required, but at least it got rid of the Pershing’s vulnerable ammunition racks. There was also a variant with a new turret designed specifically for the Pershing. Finally, there was a suggestion to install a Sherman Vc turret on a Pershing wholesale. The reduced mass could be put to good use by increasing the thickness of the armour. This variant was not used, as when a turret arrived in Fort Knox for trials it turned out that it didn’t fit on the Pershing hull.

A Firefly turret sent to the US fitted on an M4A2 tank for display purposes. Actual trials took place with an M4A3 hull.

Trials took place using an M4A3 hull. The results were not impressive. The turret was too cramped and it was hard to load the 883 mm long rounds weighing 16.8 kg. The penetration was higher than that of the American 76 mm gun, but sacrificing the crew’s comfort wasn’t worth it. The APDS ammunition that came with the gun was a good design, and the Americans used it to develop their own HVAP.

Firefly with a stinger

As powerful as it was, the APCBC shell was not the main feature of the Sherman Ic and Sherman Vc. A real ace up its sleeve was APDS (armour piercing discarding sabot) shot. With a muzzle velocity of 1200 m/s this shot could penetrate up to 200 mm of armour at 30 degrees. This was enough penetration to defeat the armour of any tank until the appearance of the German King Tiger.







Mass, kg






Muzzle velocity, m/s






Penetration at 30 degrees, point blank






Penetration at 30 degrees, 1000 yards






Characteristics of various types of ammunition compatible with the 17-pounder gun.

Trials of APDS ammunition began in late July of 1944. The first day of trials was full of surprises. The gun behaved unpredictably and precision with any ammunition worsened significantly after APDS was fired. A limited amount of rounds made available made investigation difficult. The cause was only discovered in late August. Fragments of the discarding sabot clipped the muzzle brake, which resulted in damage that negatively impacted precision. Widening the muzzle brake opening solved these issues, but it didn’t make the APDS shot fly any straighter.

Results of firing APDS (left) and APCBC (right) rounds superimposed over the silhouette of a Panther turret. The chance of hitting the tank was high enough at 800 yards (730 m), but at 1600 yards (1460 m) not a single shot would have hit its target.

The precision of the APCBC shell was satisfactory. At 2000 yards (1800 m) the dispersion did not exceed 137 cm, which was enough to hit a tank sized target. The dispersion of the APDS shot was 3-4 times greater. It was suggested that it could be used from 400 yards (370 m). Calculations showed that there was a 90% chance of hitting the turret of a Panther tank with APCBC from that distance, but only 45% with APDS. From 800 yards (730 m), the maximum distance at which using APDS was recommended, the chance of success was 57% and 22% respectively, and at 1000 yards it fell to 45% and 15%.

Odds of a penetrating hit against a Tiger or a Panther tank with the 17-pounder. These diagrams are based on calculations, in reality even a “nonpenetrating” hit could knock out a chunk of armour or cause severe cracking.

There was another problem. The high muzzle velocity made it impossible to see the tracer, which made fire correction difficult. The tracer was only visible at a distance of over 1600 yards where the precision was so bad that the chance of hitting anything was virtually zero. The reliability of the tracers also left much to be desired. 35 out of 48 tracers failed to ignite during the trial.

17-pounder APDS could penetrate the armour of German tanks even at an extreme angle, but a hit had to be scored first.

The British tank left a mixed impression. The designers managed to combine the most powerful tank gun the Allied had available with a proven chassis. However, installing the gun forced the designers to sacrifice a lot of what made the Sherman a good tank in the first place, such as reliability and crew comfort. Nevertheless, the staff at the Lulworth proving grounds described the tank as “one of the best that we tested recently”. The war was coming to an end, but first many difficult trials awaited the new British tank.

  • John Buckley, British Armour in the Normandy Campaign – Routledge, 2004
  • Mark Hayward, Sherman Firefly – Barbarossa Books, 2001
  • Documents from the Canadian Military Headquarters, London (1939-1947), RG 24 C 2
  • Library and Archives Canada image archive

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