possible, but it should be doable in England, he had liased with Colonel William Watson, who in late 1942 had returned from secondment as a British officer to Australia to work alongside Alan Chamberlain in the production of the Australian AC1 Sentinel tank program, which itself was a hybrid of the M3 Lee and Covenanter Cruiser tanks, which had successfully mounted the British 17-pounder onto the Sentinel AC1 and the 25-pounder gun onto the Sentinel AC1B. In doing so, the endeavour was taken out of the hands of the highly enthusiastic and devoted amateurs at Lulworth who had initiated it and given to professional tank developers. Design W. G. K. Kilbourn, a Vickers engineer working for the DTD, transformed their idea into the reality of the prototype of the tank that would serve the British forces from the invasion of Normandy in June 1944. The first thing Kilbourn had to fix was the lack of a workable recoil system for the 17-pounder. The 17-pounder travelled back as it absorbed the recoil of the blast. This was too long for the Sherman's turret. Kilbourn solved this problem by redesigning the recoil system completely rather than modifying it. The recoil cylinders were shortened and placed on both sides of the gun to take advantage of the width of the turret. The gun breech itself was also rotated 90 degrees to allow loading from the left rather than from on top. The radio, normally mounted in the back of the turret in British tanks, had to be moved; an armoured box (a "bustle") was attached to the back of the turret to house it, with access through a large hole cut through the turret. The next problem encountered by Kilbourn was that the gun cradle, the metal block on which the gun sat, had to be shortened to allow the gun to fit into the Firefly and thus the gun itself was not very stable. Kilbourn had a new barrel designed for the 17-pounder that had a longer un-tapered section at the base, which helped solve the stability problem. A new mantlet was designed to house this gun and the modified cradle. The Firefly had no armour or mobility advantages over the normal Sherman tank beyond the additional 13 mm of protection added to its mantlet. The modifications were extensive enough that 17-pounders intended for the Firefly had to be factory-built specifically for