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The FN5 was a gun turret designed to replace the original Barnes Wallis turrets used at the front and rear of the Wellington Mk I bomber. The same turret was later used as the nose turret of the Manchester, Stirling and Lancaster bombers, making it the most numerous British gun turret of the Second World War. In all over 22,000 FN5 turrets were produced. The FN5 was developed under conditions of great urgency in the build up to the Second World War between 1938 and 1939.
The FN5 carried two 0.303in Browning Mk II machine guns, with 1000 rounds per gun when used as a front turret and 2,000 rounds per gun as a rear turret. The guns were located to either side of the gunner, with easy access in case of blockage or other problems.
The Wellington rear turret gave no problems, but the front turret suffered from some vibration, and affected the flight characteristics of the aircraft when it was fully turned to the side. Despite these minor problems, the FN5 was used for both front and rear turrets on the Wellington IA, IC and II. It was replaced by the four gun FN20 rear turret on the Wellington III. Space was tight in the front turret, which was just above the bomb aimer’s position, and the role of bomb aimer and front gunner was sometimes combined. The FN5 equipped Wellington IA was just coming into service at the outbreak of war in 1939.
The same space problem was true of the Avro Manchester. In contrast, the Short Stirling and Avro Lancaster had plenty of space for both turret and bomb aimer. All three of those aircraft used the FN5 as a front turret, but four gun turrets at the rear, where the danger of attack was perceived to be higher.
There was not enough space in the turret for the gunner to wear his parachute and operate the turret properly, so the gunner’s parachute was attached to the fuselage just outside the turret. If the bomb aimer needed to abandon ship, he had to turn the turret to point forwards, open the door, retrieve his parachute, close the door, rotate the turret until it was fully turned to one side, then open the door and fall backwards out of the turret. The rear gunner was in the best position to do this, but the front gunner had a nervous exit as he fell past the engines! The turret could also be rotated manually from inside the aircraft, to allow the rest of the crew to rescue a trapped or wounded gunner.