Design
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.
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