it. Kilbourn had to deal with other problems. On the standard Sherman tank, there was a single hatch in the turret through which the commander, gunner and loader entered and left the tank. The 17-pounder's larger breech and recoil system made it significantly more difficult for the loader to exit quickly; a new hatch was cut into the top of the turret over the gunner's position to resolve this. The final major change was the elimination of the hull gunner in favour of space for more 17-pounder ammunition, which was longer than the original 75 mm. By October and November 1943, enthusiasm began to grow for the project. The 21st Army Group was informed of the new tank in October 1943. Even before final testing had taken place in February 1944, an order for 2,100 Sherman tanks armed with the 17-pounder gun was placed, as the Challenger programme was suffering constant delays and it was realised that few would be ready for Normandy. Even worse, it was discovered that the Cromwell did not have a turret ring wide enough to take the new High Velocity 75mm gun (50 calibres long), so it would have to be armed with the general purpose Ordnance QF 75 mm. This left the Firefly as the only tank available with firepower superior to the QF 75 mm gun in the British arsenal, earning it the "highest priority" from Winston Churchill. Name The nickname "Firefly" was adopted due to the bright muzzle flash of the main gun. It was sometimes used at unit level (brigade/regiment) war diaries from March 1944, along with "Mayfly". During the war, Shermans with 17-pounder guns were usually known as "1C", "1C Hybrid", or "VC", depending on the basic mark of the vehicle. In British nomenclature, a "C" at the end of the Roman numeral indicated a tank equipped with the 17-pounder. The name "Firefly" in period sources often refers to any vehicle with a 17-pounder gun, often the 17pdr SP Achilles M10C variant of the M10 tank destroyer. Armament The main armament of the Sherman Firefly was the Ordnance Quick-Firing 17-pounder. Designed as the successor to the British QF 6-pounder, the 17-pounder was the most powerful British tank gun of the war, and one of the most powerful of any nationality, being able to penetrate more armour than the 8.8 cm KwK 36 fitted to the German Tiger I.The 17-pounder was