When the United States Army successfully mated the chassis of the M4 Sherman to the 105mm field howitzer to create the new M7 Priest self-propelled artillery vehicle thought was given to a similar idea in devising a dedicated tank destroyer. The M4 Sherman formed the backbone armor of British and American tank actions once it was introduced and fought from the deserts of Africa through the hills of Europe and across the vastness of the Pacific. Its reach was such that thousands upon thousands were produced and the chassis served to further other battlefield required designs. US authorities then requested a dedicated tank destroyer based on the adaptable chassis of the fabled M4 Sherman series with the intent that it be fielded along with anti-tank forces to engage swathes of incoming enemy armor at distance. This would free up combat tanks to go forward and conquer alongside infantry whilst the anti-tank detachments could directly engage the enemy response or counter any break in the frontline. As such, the new tank destroyer would have to be mobile and very well-armed for the role. Armor protection would take a backseat considering the reserve role the Americans envisioned for their tank destroyer forces. Development of the new weapon system began in 1942.
The M4A2 Sherman model was therefore selected for the conversion process. The process would be handled by the Fisher Body Company and evaluations were headed by the US Tank Destroyer Board. An open-topped, rounded turret - initially developed for the abandoned T1/M6 Heavy Tank - was installed onto the body of an M4A2 Sherman and a 3-inch (76.2mm) M7 gun was fixed as primary armament. The use of a turret allowed for unfettered 360-traversal and engagement of enemies from nearly any attack angle. The TDB reviewed the Fisher offering, which was then designated as the "3-Inch Gun Motor Carriage T35", and found it lacking in some key areas. Revisions were ordered that included a modified sloped turret that provided for more ballistics protection as well as a lower profile target to the enemy. The changes were instituted to become the "3-Inch Gun Motor Carriage T35E1" prototype which sported the new angular open-topped turret we know for the M10 today. The open-topped approach was an accepted practice for such vehicles for it allowed enough "elbow" room for the gunnery crew to manage the firing functions while providing for clear vision in battlefield observation and saved on weight of additional armor. Only the driver would sit in a compartment protected from the elements and inherent dangers of the battlefield.
With the changes in tow, the design was formally accepted as the M10 GMC ("Gun Motor Carriage") in June of 1942. Production was slated to begin in September and, by this time, the M4 Sherman tank line had evolved to the point of producing a new chassis - the M4A3. It was accepted that this new chassis would also form the chassis of the equally new M10A1 variant. Some 5,000 M10s were produced - 4,993 of these from the short period of September 1942 to December 1942 - while a further 1,700 M10A1s soon joined them. The early batches stocked the inventories of awaiting US Army tank battalions which numbered 106 elements by early 1943 and each vehicle sported the 76.2mm M7 gun. 36 x M10 vehicles stocked each US battalion. The last 300 vehicles were given the 76mm M1 gun with improved penetration qualities over that of the original M7. The M10 family would go down as the most-produced American tank destroyer of the entire war with manufacture spanning 1942 to 1943 out of both General Motors and Ford vehicle plants.
Outwardly, the M10 showcased a conventional appearance save for the sharply angled, clean lines on both hull and turret. The hull was nothing more than a modified M4 Sherman body with slightly sloped side panels and a large-area sloped glacis plate. The track systems were decidedly Sherman in their length and thin appearance. The drive sprocket was held forward with the track idler at the rear. There were six rubber-tired road wheels to a track side, each paired to their respective suspension systems. The chassis was suspended by a Vertical Volute Spring Suspension (VVSS) system as in the M4 Sherman models. The turret was centered along the superstructure roof with full 360-degree unobstructed traversal, quite the departure from the German tank destroyers that made use of fixed superstructures and limited traverse main guns. However, the turret rotation was completely manual and, in practice, rather slow to respond to the rapidly changing scenarios of battle. The sides of the turret were extremely angular in their appearance while a hefty gun mantlet sat over the main gun mount base. The barrel extended out a distance ahead of the hull front and no muzzle brake was fitted. The turret was open-topped which allowed the gunnery crew the maximum amount of operating space possible. As in the M4 Sherman, the M10 was crewed by five personnel. This included the driver situated in the front left of the hull. The commander was positioned in the turret while the firing action was managed by three gunnery crew - a gun layer and two ammunition handlers.
The M10 was powered by a General Motors GM6046 6-71 Twin Diesel engine outputting 375 horsepower. This allowed the vehicle a top speed of 30 miles per hour on paved level roads and an operational range of 200 miles. Both of these numbers were good considering the vehicle's weight registering nearly 30 tons. The M10A1 production model differed not only in the M4 Sherman chassis used, but also in the installation of a Ford GAA gasoline-fueled engine. In both versions, the engine was fitted in a rear compartment (as in the M4 Sherman). Commonality of parts made for good logistics.
The main armament of American M10s was the 76.2mm Gun M7 main gun (later versions installed the improved 76mm M1 series). While lacking in the penetration capabilities of similar German (75mm), Soviet and British systems, the weapon system was generally effective. Up to 54 projectiles of 76.2mm ammunition was carried aboard. Secondary armament was the standard 0.50 caliber Browning M2HB heavy machine gun, suitable for marking targets, engaging enemy personnel and light vehicles or defending against low-flying enemy aircraft.
Once available in number, both versions of the American tank destroyer were shipped overseas to elements of the awaiting British Army. As British resources and industrial might were at their limit with other concerns, America ended up supplying much of the military industrial muscle throughout the course of the war. The M10 was known to the British in its base form as the "Wolverine" and the "Achilles" became a variant fitting the British 17-pdr (76.2mm) main gun as used on the British Sherman "Firefly" tank killer conversions. Using a locally-produced, proven weapon made sense from a logistical standpoint and did much to increase the lethality of the original American product. This version was easily identified by the addition of a muzzle brake to the barrel and came in two variants - the diesel-powered Achilles Mk IC (M10) and the gasoline-powered Achilles Mk IIC (M10A1). In the end, it was the British 17-pdr armed variants that proved the most effective M10s in the war.
M10s went to war with the British Army across both France and Italy where they proved effective. Free French forces also operated the type in their bid to retake their lands from German control. After the capitulation of Italy as an Axis power in September of 1943, it too utilized the ubiquitous M10 system for the duration of the war in its fight to remove German forces from its borders. The Polish Army received the M10 and dutifully used them against their German invaders with gusto. The Soviet Army also took delivery of M10s via Lend-Lease but it is unknown as to the value these systems held in subsequent combat against the Germans along the East Front.
In practice, the M10 was a large and heavy vehicle and suffered the fate of most of the US tank destroyers - she was lightly armored and therefore an easy target on the battlefield herself. The lack of overhead coverage for the gunnery crew also entertained the prospect of injury from artillery spray or small arms fire. It was not until enough operational experience was garnered that the idea of separate tank destroyer groups operating apart from battle tanks was deemed inefficient - essentially it was reasoned that a dedicated battle tank was needed to combat an enemy battle tank. This led to American tank destroyer groups being utilized more in the assault role used in conjunction with air support and reconnaissance forces or assigned directly as part of battle tank groups. Its 76.2mm main gun was also found to be less and less effective as the war dragged on with improvements in German armor. By the end of the war in 1945, the concept of the dedicated tank destroyer was dropped from most every major military power including the United States - tanks could now defeat enemy tanks head-on without the need for such dedicated vehicles.
Back in October of 1942, thought was given to "up-gunning" the M10 with a 90mm high-velocity gun based on a proven anti-tank field gun. However, the turret of the M10 had already proved that the existing 76.2mm mounting was enough to exceed the recommended forces. Ensuing evaluations fortified the thought that the M10 was a developmental "dead end" in terms of a more effective gun which led to the creation of a similar system - the experimental "T71 GMC". The T71 passed its evaluation period and was ushered into production in late 1943, becoming available in greater numbers by June of 1944 - to which point it was now known as the M36 GMC (nicknamed "Jackson" or "Slugger").
First combat actions of the M10 - as well as the fabled M4 Sherman tank for that matter - occurred in the North Africa Campaign where the German Afrika Corps, at one time, maintained all of the initiative. The M10 fired its first shots in anger in actions across Tunisia in 1943 where it proved a sound design against the Axis tanks being fielded at the time - lightly armed Italian medium tanks and the German Panzer III and Panzer IV medium tanks. In Europe, M10s took part in the breakout of Normandy after the D-Day landings but had a rough go of it against the newer German Panther and Tiger I series tanks which fielded considerably more armor protection - particularly along their front facings. The arrival of the newer M18 "Hellcat" series beefed up Allied tank destroyer firepower for the interim but the true replacement came at the end of 1944 when the aforementioned M36's came to being. The M36 was much more effective against the latest German tank offerings. To a limited and less favorable extent, M10s also saw service in the island-hopping campaigns of the Pacific. As the Japanese generally lacked much in the way of competent medium tanks, the M4 Sherman was the major thoroughbred in the theater. Most of the combat in the Pacific was nevertheless decided by the individual soldier and not so much the tank.
In the post-war world, M10s managed an extended existence as surplus M10s found their way to China. These were delivered sans their American main guns which provided the Chinese the opportunity to mount captured Japanese 105mm field guns instead. In essence, these became make-shift self-propelled artillery platforms of some value to the recovering Chinese Army.