The 7.7 mm (0.303 in) Vickers Gas-Operated Mk1 No.1
This gun had several official and unofficial titles. Its official service title was is shown above, Vickers called it the Class K; but in the RAF and FAA it was usually referred to as the VGO.
In the mid-thirties, gunners in the open cockpits of RAF and FAA multi-seat aircraft were still armed with the Lewis gun, but it was becoming obvious that this would have to be replaced with a more up-to-date design. Trials had just been carried out for a fixed gun, which resulted in the selection of the Colt Browning, but it was decided that a second gun was needed for the free position of bomber aircraft, and also to provide a 'second string'. At Martlesham Heath in September 1935 six types of gun were submitted for what could have been highly lucrative production contracts. The final choice was narrowed down to between the French Darne and the Vickers Class K. The French Darne was a belt-fed, gas-operated gun which, because of its short bolt stroke had a cyclic rate of 1,700 rpm. The vickers was a sturdy drum-fed gun, ideal for pivoted mountings, and because it was a simpler design, it was easier to service. Owing to the disagreement among the evaluation experts, Air Vice Marshal Dowding decided to conduct air firing tests personally, and Vickers emerged the winner. The Crayford works were given an initial order for 3,000 guns, and the first 200 were delivered in 1937.
Design History and operation
Foreseeing the need for a light machine gun for infantry, Vickers had acquiired the rights to a design by the French lieutenant Andre Berthier in 1918. However, by 1934 the British Army selected the Bren gun instead, which was based on a Czech design. The Vickers weapon had been found light and easily manouevrable, but it was prone to component failure and had a rather slow rate of fire. Vickers decided to redesign it, and a team led by Percy Higson produced a better weapon with a cyclic rate of 1,050 rpm. As the Class K, it was then ready for the 1935 RAF trials, and was given a very favourable report by the two principal armament experts at Martlesham Heath, Major H.S.V. Thompson and Captain E.S.R. Adams.
The gun was cocked by pulling back a handle on the left side. The breech-block was retained at the rear of the receiver on a projection at the back of the piston rod. When the trigger was pressed, the rod was driven forward by the force of the main spring, carrying with it the breech-block, which pushed a cartridge from the magazine into the chamber. As the piston continued to move forward, the rear of the breech-block was engaged on a sloping projection on the rear of the piston, and was forced in front of a locking shoulder on the main body. The floating firing pin was then struck by a projection on the rear of the rod, firing the cartridge. The gas pressure impinged on the head of the piston housed beneath the barrel, driving it to the rear, compressing the main spring and unlocking and withdrawing the breech-block. The rearward movement of the block extracted the case and ejected it into a container at the side of the gun. A saftey catch was incorporated into the hand grip which rendered the gun safe when not in use. After final assembly at Crayford, the guns were proof-fired with ammunition containing 25 per cent more charge than normal. They were then tested for automatic fire, first in a horizontal position, then at an angle of 90 degrees.
The pistons in the early production guns tended to fail after firing 1,000 rounds or so, Higson was able to correct this and other small faults. The modified gun proved to be highly efficient, the minimum life of any component being 10,000 rounds - a great improvement over the Lewis gun, for which replacement parts had to be carried in the aircraft. The 47-round magazine was dropped in favour of a 100-round drum, which gave a more realistic ammunition supply. Higson stressed the modest recoil, the absence of external moving parts, and the fact that the gun could be dismantled in a few seconds with no other tools than an empty cartridge case and a pen-knife.
Initially, armourers had trouble tensioning the spring of the 100-round magazine, but limiting the load to 97 rounds overcame the problem. Stoppages did occur, the most frequent cause being defective ammunition and badly filled magazines. Lack of maintenance could lead to short or broken firing pins, defective extractors and broken springs. Perhaps the most disturbing experience was not a gun which stopped firing, but a gun which would not stop!
The gun proved so successful that a cable-operated version was specified for all turrets mounting a single gun and (in the case of Blenheims and Beauforts) twin guns. Vickers produced a belt fed fixed version, for which ammunition was supplied from 300 or 600 round tanks. Some early Blenheim 1 aircraft had the fixed version of this gun, but almost all were used on pivoted mountings and turrets. The Bristol Bombay nose and tail turrets each had a single gun, as did the front turrets of the Whitley, Sunderland and Lerwick flying-boats. When the gun was fired from these turrets the piston rods often broke. It was eventually found that the part of the gas cylinder outside the turret was chilled by the slipstream, whereas the section inside the turret soon reached a high temperature after a few bursts were fired, causing the piston to seize. The cylinder diameter was increased but, although this reduced the failure rate, gunners of No.5 Group (Wellingtons) were advised to carry spares for the side hatch guns. Being self-contained and needing no belt boxes, the gun was used in the nose position of all later Halifax bombers, and Sunderland and Beauforts used it as additional armament to power turrets. Pivoted guns were usually sighted by 51 mm (2 in) ring-and-bead sights, but Mk 111 reflector sites were also used on both free-mounted and turret installations.
Many army units used the VGO. The Long Range Desert Group of the 8th Army, as well as the SAS, found that it was far less liable to stoppages than the Bren, and it was also used by the Indian Army (Vickers Berthier or VB Mks 1, 2, 3 & 3B) instead of the Bren.
Details of the 7.7 mm (0.303 in) Vickers Gas-Operated
Calibre: 7.7 mm (0.303 in)
Weight: 9.5 kg (20.5 lb)
Muzzle Velocity: 732 m/sec (2,400 ft/sec)
Cyclic Rate: 950-1,100 rpm
Maximum Range: 914 m (1,000 yds)
Weight of Bullet (Mk VIII): 11.34 grams (0.4 oz)
Ammunition Feed: 47-(later 97-) round drum
Length: 1,016 mm (40 in)
Rifling: Five grooves, left hand.
The above text and photos were taken from "British Aircraft Armament Vol 2: RAF Guns and Gunsights", by R. Wallace Clark.
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Last edited by Robert Hurst on 06 Mar 2003 12:38, edited 3 times in total.