The earlier introduction of US M3 Grant and M4 Sherman tanks placed a dual purpose 75 mm gun into British and Commonwealth service. The 75 mm could fire a more effective HE shell than the 6-pounder at the cost of a reduction in armour-piercing performance. The 6-pounder firing solely AP shells was a retrograde step. Once the Vickers 75 mm HV gun was seen to be too big for the Cromwell turret, work was begun in December 1942 on the Ordnance QF 75 mm (a development of the 6-pounder that fired US ammunition) for fitting to British tanks. Mark IV Cromwells were delivered with 75 mm guns from November 1943.