Ordnance QF 75 mm

Ordnance QF 75 mm
Ordnance QF 75 mm
	Gun on Cromwell tank at Overloon War Museum, Netherlands
Type	Tank gun
Place of origin	United Kingdom
Service history
Used by	British Empire
Wars	Second World War

The Ordnance QF 75 mm, abbreviated to OQF 75 mm, was a British tank-gun of the Second World War. It was obtained by boring out the Ordnance QF 6 pounder ("6 pdr") 57-mm anti-tank gun to 75-mm, to give better performance against infantry targets in a similar fashion to the 75mm M3 gun fitted to the American Sherman tank. The QF came from "quick-firing", referring to the use of ammunition with the shell and propellant in a single cartridge. 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 (40-mm), and then the larger 57mm 6 pounder. These guns were designed to fire armour-piercing shot, small high-velocity solid rounds that were effective against tanks, but did little damage to groups of infantry or soft targets like trucks. Some tanks operating in the infantry support role were given howitzers firing HE shells, such as early models of the Churchill and CS (Close Support) versions of the Matilda II. The decision to equip British tanks with a gun capable of firing HE shells at soft targets was taken by the War Office.

A HE shell for the 6 pounder was in production by the start of the Tunisia Campaign and available in large numbers in the Italian Campaign. However, the round lacked sufficient explosive power. The power of the US 75mm HE round used in the 75mm M3 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.

Vickers was working on a high velocity 75 mm gun to be fitted to British tanks. This took the cartridge case of the 3 inch 20 cwt anti-aircraft gun mated to the US 75mm AP and HE shell. With a barrel length of 50 calibres, it would have had about twice the muzzle energy of the US 75mm gun. However, the final gun turned out to be too big to fit into the tank that it had been expected to fit.[1]

It was noticed that the US 75mm cartridge was almost the same diameter as the British 6 pounder case. Instead 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 logistics since both British-made and American-made 75mm 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. In the Battle of Villers-Bocage, Cromwell tanks with the 75mm were outgunned by Tiger tanks of the 101st SS Heavy Tank Battalion.

Though the 75mm had a good HE shell, it was still thought that a more powerful close support weapon was needed and as such the 95mm howitzer was agreed for a limited number of tanks.[2]

Ammunition

Loading ammunition into a Churchill tank, Normandy July 1944

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[nb 1] with either the M48 or M54 fuze

Bursting charge was 1.49 lb (0.68 kg) TNT or 1.36 lb (0.62 kg) 50/50 Amatol or 1.52 lb (0.69 kg) trimonite.[nb 2] 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 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 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, and 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".

British tank guns of the Second World War
Gun	Caliber	Shell weight	Muzzle velocity	Muzzle energy kJ
2 pdr	40 mm (1.6 in)	(AP/T shot) 2.7 lb (1.2 kg)	2,650 ft/s (810 m/s)	295
6 pdr	57 mm (2.2 in)	(AP shot) 6.3 lb (2.9 kg)	3,000 ft/s (910 m/s)	1,100
75 mm		14.9 lb (6.8 kg)	2,050 ft/s (620 m/s)	1,300
17 pdr	76.2 mm (3.00 in)	17 lb (7.7 kg)	2,950 ft/s (900 m/s)	3,100

References

Notes

full name for the round was "Shell, fixed, HE M48, normal charge" followed by the fuze specification
80% ammonium nitrate, 10% TNT, 10% aluminium

Citations

An Alternative 1930s British Tank Gun - And Its Successor Anthony G Williams
Cromwell Vehicle History and Specification 1983 HMSO p. xi

Bibliography

US Document WO 219/2806, Appendix G to SHAEF/16652/GCT/Arty,
"Fire and Movement", Bovington Tank Museum
Churchill Tank - Vehicle History and Specifications 1983 HMSO ISBN 0-11-290404-1

External links

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[3] ull name for the round was "Shell, fixed, HE M48, normal charge" followed by the fuze specification

[4] 80% ammonium nitrate, 10% TNT, 10% aluminium

[1] An Alternative 1930s British Tank Gun - And Its Successor Anthony G Williams

[2] Cromwell Vehicle History and Specification 1983 HMSO p. xi

British Commonwealth artillery of World War II
Tank guns	QF 2-pounder QF 3-pounder QF 6-pounder QF 75 mm QF 3-inch howitzer QF 17-pounder 77 mm HV QF 95 mm howitzer
Anti-tank guns	QF 2-pounder QF 6-pounder QF 17-pounder
Field guns and howitzers	75 mm Gun M1917 QF 18-pounder 25-pounder Gun-Howitzer 25-pounder Short QF 4.5-inch howitzer
Medium and heavy guns and howitzers	BL 4.5-inch Medium Field Gun BL 60-pounder gun BL 5.5-inch Medium Gun BL 6-inch howitzer BL 6-inch gun Mk XIX 155 mm Long Tom BL 7.2-inch howitzer BL 8-inch howitzer BL 9.2-inch howitzer 240 mm howitzer M1
Mountain guns	75mm Pack howitzer M1 3.7-inch mountain howitzer
Mortars	SBML 2-inch mortar ML 3-inch mortar ML 4.2-inch mortar
Anti-aircraft weapons	Z Battery 20 mm Oerlikon 20 mm Polsten QF 1½-pounder Mk III QF 2-pounder naval gun Bofors 40 mm QF 3-inch QF 3.7-inch QF 4.5-inch Mk II QF 5.25-inch Mk II
Coast defence	QF 6-pounder 10 cwt QF 12-pounder QF 4.7-inch Mk I–IV BL 6-inch Mk VII & Mk XXIV BL 7.5-inch Mk VI BL 8-inch Mk VIII BL 9.2-inch Mk X BL 14-inch Mk VII BL 15-inch Mk I
Railway artillery	BL 9.2-inch Mk XIII railway gun BL 12-inch Mk V railway howitzer BL 13.5-inch Mk V railway gun BL 18-inch railway howitzer