The 17 pounder, Self-Propelled, Achilles was a British variant of the American M10 tank destroyer armed with the powerful British Ordnance QF 17 pounder anti-tank gun in place of the standard 3" (76.2 mm) Gun M7. With a total of 1,100 M10s converted, the 17 pdr SP Achilles was the second most numerous armoured fighting vehicle to see service armed with the 17 pounder gun, behind the Sherman Firefly.
The name "Achilles" was officially a designation applied to both the 3" gun and 17 pounder versions (as Achilles I/II and Achilles Ic/IIc respectively) but was little used during the Second World War; at the time, the vehicle was called 17pdr M10, or 17pdr SP M10, or even occasionally, "Firefly". It has since become identified almost exclusively with the 17 pounder version.
In the wake of Germany's successful 1939–41 campaigns, US armour doctrine had incorporated the idea of fast, lightly armoured vehicles carrying high velocity anti-tank guns as the best way to deal with the fast moving armour spearheads of the German Blitzkrieg. The M10 was based on the chassis of the M4 Sherman but carried thinner although more sloped armour in order to comply with the high speed requirement for the tank. At the same time, the British had been examining the possibility of designing a low-silhouette self-propelled tank destroyer, preferably with a 360-degree traversing turret, with armour that would be able to resist the German 50 mm at 800 yards and mounting the 17 pounder. However, with the arrival of the M10 on the battlefield in late 1942, British plans for a turreted self-propelled gun were cancelled.
The M10 was first made available to the British in 1943. These vehicles were open-topped and mounted a 3-inch American gun, which was significantly more powerful than the Ordnance QF 6 pounder that was mounted on British tanks of the period and was of equal power to the 7.5 cm KwK 40 used by the Panzer IV and Sturmgeschütz III. When introduced into service in late 1942, the thin but sloped armour of the M10 provided good protection against the standard 50 mm gun mounted on most German tanks and anti-tank guns, and the 3-inch (76 mm) gun was able to easily defeat all German armour except for the handful of Tigers deployed against the Western Allies.
The 17 pdr SP Achilles was basically a modified M10, the main difference being the gun. The main armament of the Achilles was the Ordnance QF 17 pounder, a gun with greatly superior anti-tank performance over the standard American 3" anti-tank gun. The single top mounted M2 Browning HMG was retained.
The 17 pounder mounted on the Achilles was able to penetrate some 140 mm of armour at 500 m (550 yd) and 131 mm at 1,000 m (1,100 yd) using standard Armour Piercing, Capped, Ballistic Capped (APCBC) ammunition impacting at a 30-degree angle. When supplied, Armour-piercing discarding sabot (APDS) ammunition could penetrate some 209 mm (8 inches) of armour at 500 metres and 192 mm at 1,000 metres at a 30-degree angle. In comparison, the 3-inch (76 mm) gun on the standard M10 using the same type of ammunition (APCBC) would penetrate 98 mm of armour at 500 m at a 30-degree angle, and 88 mm of armor at 1,000 meters at a 30-degree angle. Only with High Velocity Armour Piercing (HVAP) ammunition did the 3-inch (76 mm) gun compare with the 17 pounder, the ammunition being able to penetrate 140 mm at 500 m at a 30-degree angle, and 127 mm at 1,000 m at a 30-degree angle. However, HVAP ammunition was in very short supply and it only just compared with the standard 17 pounder ammunition that was available in huge amounts for the British.
The 17 pounder required a counterweight fitted behind the muzzle brake on its long barrel. This gave the Achilles a distinctive appearance compared to the M10 and there were attempts to disguise this by painting the brake and counterweight.
The only other change carried out on the Achilles was the addition of 17 mm (0.67 in) thick armour plates welded to the front and sides of the M10 to increase armour protection, as well as a 20 mm thick shield fitted to the top of the turret to provide protection from overhead threats that resulted from the M10 having an open top turret.
The desire to mount the 17 pounder on the M10 was governed by the degree of difficulty involved in mounting the 17 pounder on the tank itself. Luckily for the British, the initial batches of M10s had an easily modified gun mounting to facilitate the future replacement of the older 3 inch M7 gun with the newer 76 mm gun M1. This gun mounting design allowed for the British to replace the 3 inch gun with the 17 pounder gun. The British took delivery of some 845 vehicles in 1943, but of the second version of the M10, only the T71 mark could carry the 17 pounder, the T70 mark being designed to only allow the lighter American 76.2 mm M1 gun.
The British had planned to convert some 1,000 M10s into 17pdr armed variants for Normandy, but for some reason conversions were not started until April 1944. By D-Day, only some 124 M10s had been converted; however, the number of conversions post D-day increased and by the end of the year 816 M10s had been converted, 152 vehicles in November alone. However, the low numbers at D-day meant that many British units went ashore fielding standard M10s rather than 17 pounder armed Achilles, and losses in Achilles units could at times be hard to replace, the crews receiving regular 3" armed M10s as replacements for their lost 17 pdr Achilles much to their dismay.
As a self-propelled anti-tank gun, Achilles as well as standard M10s were distributed to and operated by the regiments of the Royal Artillery. Around 1,650 M10s were received by the British during the war, of these 1,100 were converted to the 17 pdr by the end of the war.
Unlike the Americans, who saw the M10 as a tank hunter once it became clear that the Germans had gone on the defensive, the British still viewed the Achilles as a mobile anti-tank gun. The standard anti-tank gun used in infantry units in the British Army was the 6 pounder anti-tank gun, a small, light gun able to defeat the more common German Panzer IV and Sturmgeschütz with regular ammunition but not the heavier Tigers and Panthers. The next generation British gun, the 17 pounder anti-tank gun, was able to deal with Tigers and Panthers but had a far longer emplacement time than the 6 pounder due to its larger size, often taking a day to fully emplace and prepare for action.
As a result, the British used the Achilles as a quickly deployable anti-tank gun, able to reinforce a position taken by infantry and engage counter-attacking German forces while the slower towed 17 pounders were pulled up and dug in for a more long-term defensive presence. This had the advantage of mitigating the weak armour protection of the Achilles as being used defensively usually allowed it to fire the vital first shot. This was in line with the original design concept of the vehicle, intended to blunt German "blitzkrieg" attack tactics. The M10/Achilles turret had an extremely slow manual only turret rotation, a limited tactical disadvantage when the tank was used only in a defensive role. As such, they had an advantage over German Stugs, which had no turret rotation. However, the Stug's low profile and heavy armor were significant assets that allowed it to be successful in its different tactical environment. Usually, the only time the British used the M10 and Achilles offensively was in support of Churchill tank units, which lacked the associated 17 pdr-armed tanks that Sherman and Cromwell tank units had.
Achilles went ashore on D-Day, equipping units of the Royal Artillery and Royal Canadian Artillery in Armoured Division or Corps Anti-tank Regiments. A typical A/Tk Regiment would have 4 Batteries, 2 x Towed 17 Pdr Batteries, 1 x Achilles and 1 x M10 Battery. The M10 Battery was replaced by a second Achilles Battery as more vehicles became available. Perhaps the most successful action of the Achilles was conducted by B troop, 245th Battery, 62nd Anti-Tank Regiment, Royal Artillery attached to the Hamilton Light Infantry during Operation Charnwood. South of Buron, a counterattack by a mixed force of Panzer IV and Panther tanks of the 12th SS Panzer Regiment was defeated by Achilles and 17-pounder anti tank guns of 245th Battery, 62nd Anti-tank Regiment. Thirteen German tanks were destroyed in one of the most successful antitank engagements of the campaign, for the loss of four self-propelled guns and a further four damaged.