The 17 pdr SP Achilles was basically a modified M10, the principal difference being the gun.
The main armament of the Achilles was the Ordnance QF 17 pounder, a substantially more powerful gun than the 3 in (76.2 mm) M7 mounted on the standard M10.
The single top-mounted .50 calibre M2 Browning heavy machine gun 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, though the accuracy of the APDS round was poor during the war.
 In comparison, the M7 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 armour at 1,000 metres at a 30-degree angle.
Only with High Velocity Armour Piercing (HVAP) ammunition did that 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, whereas the standard 17 pounder ammunition was available in huge quantities 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 comparatively short-barrelled, brakeless, entirely straight barrel of the M10.
 Once the Germans discovered the effectiveness of the Achilles' gun on the same M10 carriage as the much less powerful M7 gun, attempts were made to disguise the 17 pounder by painting its brake and counterweight.

Two other material changes were made to the Achilles: the addition of 17 mm (0.67 in) thick armour plates welded to the front and sides of the M10 to increase armour protection; and the M10's open turret was fitted with a 20 mm thick shield to provide protection from overhead threats.
The 17 pdr SP Achilles was basically a modified M10, the principal difference being the gun. The main armament of the Achilles was the Ordnance QF 17 pounder, a substantially more powerful gun than the 3 in (76.2 mm) M7 mounted on the standard M10. The single top-mounted .50 calibre M2 Browning heavy machine gun 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, though the accuracy of the APDS round was poor during the war.  In comparison, the M7 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 armour at 1,000 metres at a 30-degree angle. Only with High Velocity Armour Piercing (HVAP) ammunition did that 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, whereas the standard 17 pounder ammunition was available in huge quantities 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 comparatively short-barrelled, brakeless, entirely straight barrel of the M10.  Once the Germans discovered the effectiveness of the Achilles' gun on the same M10 carriage as the much less powerful M7 gun, attempts were made to disguise the 17 pounder by painting its brake and counterweight.
Two other material changes were made to the Achilles: the addition of 17 mm (0.67 in) thick armour plates welded to the front and sides of the M10 to increase armour protection; and the M10's open turret was fitted with a 20 mm thick shield to provide protection from overhead threats.
17pdr SP Achilles