Ordnance QF 75 mm
The Ordnance QF 75 mm is a British tank gun used during World War II. The basis of the gun was essentially a 6-pounder gun with a new 75 mm barrel, firing the same ammunition as the American 75 mm guns on their M4 Sherman. It was easily mounted onto tanks that used the 6-pounder due to the similar mountings.
The Ordnance QF 75 mm is very similar in ballistic performance to the American 75 mm Gun M3. It is for all intents and purposes identical to the American gun using the default M72 AP round. However, the British M61 APCBC round is inferior to the American one due to the lack of explosive filler, thus it has rather disappointing behind-armour effects compared to the American version.
The primary advantage of the M61 round is its better performance against sloped armour. It also has a slightly larger projectile mass for better post-penetration fragmentation. However, it pays for this with its noticeably worse performance against vertical armour. On some of the lower rank tanks such as the Cromwell Mk V, it may be worth switching to the M61, but the Churchill Mk VII cannot afford the lower penetration against the opposition it will meet, and thus may be better off using the default M72.
Due to the QF 75 mm's faults, it would be more preferable to move towards the 6-pounder gun, which trades a lighter projectile for a significantly higher rate of fire, greater variety in ammunition, and much higher penetration.
Guns of comparable performance
- American 75 mm Gun M3 found on the Shermans.
- German 7.5 cm KwK 37 on the early Panzer IVs.
- Soviet 76.2 mm gun on the T-34s.
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The previous two main guns used on British tanks, the 2-pounder and the 6-pounder, performed well against armored targets, but lacked the necessary high-explosive power to fight against softer targets such as infantry and especially dug-in anti-tank guns. This often forced British tanks to resort to using machine guns instead, leaving them massively out-ranged by German high-velocity anti-tank guns in the North African Campaign.
One way to solve this was to complement these tanks with specialised close support (CS) tanks armed with various howitzers, but this was hardly an ideal state of affairs. An attempt to develop a "dual-purpose" gun was undertaken by the War Office to better arm their tanks. While a high-explosive shell was available for the 6-pounder by the time the Tunisia Campaign started, the shell lacked sufficient power compared to the American 75 mm high-explosive shells. The 75 mm's performance in battle with its high-explosive power was found to be superior to any other guns in British service to such an extent that, during the Italian Campaign, several Churchill tanks were modified to use the Sherman's 75 mm guns. It was even proposed that the British simply buy the American 75 mm M3 guns to fulfill the demands, but this idea was discarded.
In 1942, there were three new tank guns under development in Britain to arm the next generation of British tanks: the Royal Arsenal's 8-pounder and Vickers' 12-pounder and high velocity 75 mm guns. On one hand, the first two proved to be an insufficient improvement over the 6-pounder and were dropped. The high velocity 75 mm, on the other hand, would not fit in any existing British tank turret, particularly that of the Cromwell cruiser tank. During the design phase, it was realized that the diameter of the 75 mm shell casings were similar in size to the 6-pounder's 57 mm rounds, so a simple expedient from Vickers was to bore out the 57 mm barrel on the 6-pounder into 75 mm, while modifying the breech to chamber the American ammunition. By May 1943, it was confirmed that Vickers' high velocity 75 mm gun would not fit in the Cromwell's turret, leaving the Ordnance QF 75 mm as the only viable option for the tank to see service with in combat.
The OQF 75 mm gun saw usage mostly on the Cromwell cruiser tank and the Churchill Mk.VII infantry tank, though some Valentine tanks also saw conversion into this gun as well. They saw service in the Italian and Normandy Campaign and were used until the end of World War II. Some tank units, except the Cromwell units, retained the 6-pounder due to its superior anti-tank capability over the 75 mm gun with the availability of APCR and APDS ammunition. Statistically, the armor-penetration capabilities of the 75 mm gun was the worst compared to all of the guns available in the British inventory at the time. However, the gun provided superior high-explosive power, to which was used to its greatest extent against the targets the British came upon in the battlefield and was seen as a worthwhile trade-off for the reduced anti-armor performance.
- Fletcher 2006, The Guns
- TheTankMuseum 2017
- Fletcher, David. Cromwell Cruiser Tank 1942-1950 Great Britain: Osprey Publishing Ltd., 2006. Kindle Edition
- TheTankMuseum. Tank Chats #32 Cromwell YouTube, 24 Feb. 2017. Web. 03 Aug. 2017. Video.