One of Rommel's most successful tactics in the Western Desert was to use his own tanks to lure a British counter attack onto his very effective anti-tank guns. Early British Tanks did not have guns capable of firing HE shells (only 2x close support (CS) tanks with "howitzers" were available to each regiment) and therefore the British Armour did not have an effective anti-anti-tank gun capability. The M4 Sherman gave the British armour that HE capability...however the 17 pdr, although it could shoot an HE shell, was not very effective in this role - the shell casing had to be very thick and heavy to withstand the high velocity of the gun, and weight of explosive in the HE shell was much reduced.
However, several methods were tried to fit the 17 pdr into other tanks...the main problem being the diameter of the turret ring and the ability to fit the tank onto railway running stock. In the Firefly, the 17 pdr had to be mounted sideways to get the gun to fit inside the turret and was therefore awkward to load. It was not until the Comet, arrived in spring 1945 that the British armour finally got an effective mount for the 17 pdr.