While developing the Challenger tank, the British devised a conversion for their US-supplied M4 Sherman tanks to mount the 17-pounder. This was applied in sufficient numbers to put them into service in time for D-Day as the Sherman Firefly. The gun was a modified design that was produced specifically for the Firefly. The breech of the gun was rotated 90 degrees to fit inside of the height of the turret, i.e. it lay on its side. An additional box was welded to the back of the turret to take the radio, which was moved to allow for the breech and its recoil. Additionally, a new recoil mechanism, based on that of the 6-pounder was developed to further shorten the recoil, while also shortening the gun cradle. Production of the Challenger was cancelled, and more Shermans were converted until about 50% of Shermans in British service were Fireflies.