of having to take the American gun to be fitted en masse into modified British tanks, the Royal Ordnance factory modified the 6-pounder design by boring out the barrel and adapting the breech to fire the US round. The resulting gun could then be fitted without redesigned tank mountings and dramatically simplified supply since both British-made and American-made 75 mm guns could use the same ammunition. It gained British tanks a good HE shell but came with an inferior anti-tank round, proving troublesome against heavily armoured German tanks. Though the 75 mm had a good HE shell, it was still thought that a more powerful close support weapon was needed and the QF 95 mm howitzer was agreed for a limited number of tanks. Ammunition The QF 75 mm used US ammunition. The shells were "fixed" ammunition, the shell cartridge and projectile being joined together as a complete round. Shell HE M48 with either M48A2, M54, M51A4, or M21A4 fuze. Bursting charge was TNT or 50/50 Amatol or trimonite. The M48 fuze could be set for impact detonation ("Superquick") or delayed detonation; when in "Superquick" setting the delay would set the shell off if the impact did not set off the fuze. The M54 round had variable delay; the fuze starting to burn at the instant of firing the round. Shot APC M61, with tracer in the base An armour-piercing capped projectile with a thin ballistic cap ("windshield") for better aerodynamics. Shot AP M72, with tracer in the base An entirely solid armour-piercing projectile. Service The ROQF 75 mm was chiefly used on the Churchill and Cromwell tanks. The weapon was used in Italy and in the Normandy invasion (and possibly in Burma against the Japanese) until the end of the war. While the 75 mm was a conversion from the 6-pounder, some units retained a number of 6-pounder-gunned tanks, due to its superior anti-tank firepower over the 75 mm, especially as the 6-pounder could use more effective APCR and APDS rounds. Externally, the gun was nearly identical to the 6-pounder gun. The 14.9 lb (6.76 kg) HE shell fired at 2,050 ft/s (625 m/s) was found to be the best available and superior to that of the 6-pounder, M7 3 in and 17-pounder, all chiefly anti-tank guns. Against armour, its AP shell was the worst, penetrating only 68 mm of RHA at 500 yards (460 m) and a 30-degree