The Sherman Firefly was a tank used by the United Kingdom and some armoured formations of other Allies in the Second World War. It was based on the US M4 Sherman, but was fitted with the more powerful 3-inch (76.2 mm) calibre British 17-pounder anti-tank gun as its main weapon. Originally 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.