By early 1940 it was becoming apparant that the 2 pdr had had its day, this was predicted in 1938 when work began on its succesor, the 6 pdr Ordnance Quick Firing gun. Unfortunately the first tanks fitted with it did not arrive till December 1941. One major issue was the older tanks could not easily be upgraded to the larger weapon. The 6 pdr was a 57 mm gun firing a 2.9 kg shell, armour penetration for standard AP ammunition was 81mm of RHA at 30 degrees and 500 metres. By this time tank armour had increased to 45 to 60mm, often German tanks were fitted with applique armour on top of the base protection. One major problem with the new gun was it only fired solid AP shot, experience in the western desert using the American sherman had shown the advantages of a weapon capable of firing both AP, HE and Smoke. British tanks still needed a number of vehicles to be equipped with a howitzer, this was now a 3 inch weapon capable of firing both smoke and HE. The British developed the 75mm Ordnance Quick Firing gun, basically a copy of that fitted to the Sherman tank. It fired American ammunition including HE. The gun was 75 mm bore firing a 6 kg projectile, armour penetration with standard AP was reduced to 68mm of RHA inclined at 30 degrees at a range of 500 metres. In some ways a retrograde step but experience in the desert was that anti-tank guns were as big a danger as enemy tanks, the HE round made destroying them much easier.
The illustrations below show a 6 pounder AP round and a 75mm AP round . .