An HE shell for the 6 pounder was in production by the time of the start of the Tunisian campaign and available in large amounts in the Italian Campaign. However, the round lacked sufficient explosive power. The power of the US 75 mm HE round used in the M3 75 mm was found to be markedly superior, and a number of Churchills in operation in Italy had guns scavenged from Sherman tanks and fitted to their turrets to give the Churchill NA75 (NA coming from "North Africa" where the conversions were carried out). Approximately 200 were converted in this way. Instead of looking to take the American gun to be fitted en masse into British tanks, the Royal Ordnance modified their 6 pdr design by boring out the barrel and adapting the breech to fire the US round. The resulting gun was effective but although gaining a good HE shell they had an inferior anti-tank round and this proved troublesome against the well armoured German tanks. In the Battle of Villers-Bocage Cromwell tanks with the 75 mm were no match in battle with a lone Tiger tank.
Externally the gun was nearly identical to the 6 pounder gun. The 14.9 lb (6.76 kg) HE shell fired at 2,050 ft/s (625 m/s) was found to be the best available - superior to that of the 6 pounder, M7 3 in and 17 pounder all chiefly anti-tank guns. However, against armour its AP shell was the worst, penetrating only 68 mm of RHA at 500 yards (460 m) and a 30 degree angle of attack, whereas the AP shells of the others penetrated between 57 mm and 76 mm in Normandy during 1944. The AP shell for the 75 mm gun was a 15 lb (6.8 kg) projectile with a couple of ounces (60 g) of HE filling propelled by a 2 lb (900 g) charge to 2,000 feet per second (610 m/s). In British service the AP shell was used without its explosive filling and as such was referred to as "AP Shot M61".
A much improved anti tank shell was developed. This was a APCBC (Armour Piercing Capped, Ballistic Cap) shot design. It had a muzzle velocity of 2,600 ft/s (790 m/s) as opposed to 2,030 ft/s (620 m/s) for the older M61 APC shell. US documents1 showed the round as available for both the M3 75 mm and ROQF 75 mm. Other sources2 indicate the shell saw use by the British only. The performance of the new shell was a vast increase: penetrating 102 mm at 500 yards (460 m) at 30 degree angle of impact. Actual availability and usage of this round is unclear. According to some British tables M72 AP had a penetration performance of 74 mm at 100 yards and 62 mm at 500 yards, so it is possible the 30 degrees angle of impact is a transposition error for 0 degrees. This is interesting as US M72 has a penetration performance of 76 mm at 100 yards.
|Gun||Shell weight||Muzzle velocity|