Sherman Firefly. The Sherman Firefly was a medium tank used by the United Kingdom and some armored formations of other Allies in the Second World War. It was based on the US M4 Sherman but was fitted with the more powerful British 76.2 mm (3.00 in) caliber 17-pounder anti-tank gun as its main weapon. Conceived as a stopgap until future British tank designs came into service, the Sherman Firefly became the most common vehicle mounting the 17-pounder in the war. The British Army made extensive use of Sherman tanks, but they expected to have their own tank models developed soon, so the idea of mounting the 17-pounder in the Sherman was initially rejected. However, through the efforts of two persistent British officers, government reluctance was eventually overcome, and the Firefly went into production. This proved fortunate, as the Cruiser Mk VIII Challenger and Cruiser Mk VIII Cromwell tank designs experienced difficulties and delays. After the problem of getting such a large gun t...