Ordnance QF 75 mm

The Ordnance QF 75 mm, abbreviated to OQF 75 mm, was a British tank-gun of the Second World War. It was used instead of the Ordnance QF 6 pounder ("6 pdr"), an anti-tank gun, to give better performance against infantry targets in a similar fashion to the 75 mm gun fitted to the American Sherman tank. The QF came from "quick-firing". The gun was also sometimes known as ROQF from Royal Ordnance (the manufacturer) Quick-Firing.

Development
Prior to the introduction of the ROQF 75 mm, British tanks had been equipped with guns such as the QF 2 pounder, and then the larger QF 6 pdr, that fired Armour Piercing shot - good against tanks but ineffective against groups of infantry. Some tanks operating in the infantry support role were given guns firing HE shells, e.g. early models of Churchill tank and CS ("Close Support") versions of the Matilda II. The decision to equip British tanks with an HE shell firing gun for "soft" targets like infantry and, importantly, anti-tank guns was taken by the War Office.

An HE shell for the 6 pounder was in production by the time of the start of the Tunisia Campaign and available in large amounts in the Italian Campaign. However, the round lacked sufficient explosive power. The power of the US 75 mm HE round used in the M3 75 mm was found to be markedly superior, and a number of Churchills in operation in Italy had guns scavenged from Sherman tanks and fitted to their turrets to give the Churchill NA75 (NA coming from "North Africa" where the conversions were carried out). Approximately 200 were converted in this way.

Instead of looking to take the American gun to be fitted en masse into modified British tanks, the Royal Ordnance modified their 6 pdr 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. It was effective but although gaining a good HE shell they had an inferior anti-tank round and this proved troublesome against the minority of heavily armoured German tanks. In the Battle of Villers-Bocage Cromwell tanks with the 75 mm were outgunned by Tiger tanks of the 101st SS Heavy Tank Battalion.

Though the 75 mm had a good HE shell, it was still thought that a more powerful close support weapon was needed and as such the Ordnance QF 95 mm howitzer was agreed for a limited number of tanks.

Ammunition
The QF 75mm used US ammunition. The shells were "fixed" ammunition, the shell cartridge and projectile being joined together as a single complete round.


 * Shell HE M46 with either the M48 or M54 fuze
 * Bursting charge was 1.49 lb TNT or 1.36 lb 50/50 Amatol or 1.52 lb 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 didn't set off fuze. The M54 round had variable delay; the fuze starting burning at the instance of firing the round.


 * Shot APC M61, with tracer in the base
 * A armour-piercing capped projectile with a thin ballistic cap ("windshield") for better aerodynamics.


 * Shot AP M72, with tracer in the base
 * an entirely solid projectile

Service
The ROQF 75 mm was chiefly used on the Churchill and Cromwell tanks. The weapon was used in Italy and 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 the even 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 - superior to that of the 6 pounder, M7 3 in and 17 pounder all chiefly anti-tank guns. However, against armour its AP shell was the worst, penetrating only 68 mm of RHA at 500 yards (460 m) and a 30 degree angle of attack, whereas the AP shells of the others penetrated between 57 mm and 76 mm in Normandy during 1944. The AP shell for the 75 mm gun was a 15 lb (6.8 kg) projectile with a couple of ounces (60 g) of HE filling propelled by a 2 lb (900 g) charge to 2,000 feet per second (610 m/s). In British service the AP shell was used without its explosive filling and as such was referred to as "AP Shot M61".

Similar named guns
In the First World War some French 75 mm field guns used by the British forces received the designation Ordnance QF 75 mm Mk I. And in the early part of the Second World War some American 75 mm Gun M1897A3 field guns were purchased from the US. These were referred to as Ordnance 75 mm.