The Sherman Firefly was a tank used by the Commonwealth and Allied armoured formations in the Second World War. It was based on the US M4 Sherman but it was fitted with the powerful 3-inch (76.2 mm) 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 war II.