Taken from page 80 of Spitfire - Flying Legend, Published by Osprey Aerospace:
Gyro Gunsight
In his volume Late Marque Spitfire Aces 1942-45 (published by Osprey in 1995), Dr Alfred Price detailed just how the Mk II Gyro Gunsight, developed by the Royal Aircraft Establishment at Farnborough, and put to good effect by "Sammy" Sampson to achieve his fourth (and last) kill of the war, worked in combat;
'This device was a great improvement over the GM 2 reflector sight, and operated on the principal that if a fighter pilot followed an enemy aircraft in a turn and held his gunsight on the latter, his rate of turn was proportional to the deflection angle required to hit the target. A gyroscope measured his rate of turn, and tilted a mirror which moved the position of the sighting graticule to show the required deflection angle. The required deflection varied with range, however, so the gunsight incorporated a simple system of optical rangefinding. Before the engagement the pilot set the aproximate wingspan of the enemy aircraft. As he closed on his foe, the pilot operated a control mounted on the throttle arm which altered the diameter of the sighting graticule so that the size matched the wingspan of the enemy aircraft. Since the wingspan of the target aircraft had been set on the sight, the adjustment of the graticule "told" the gunsight the range of the target. An analogue computer in the gunsight worked out the correct point ahead of the target at which the pilot should aim in order to score hits.
'Once fighter pilots got used to the new sight, and learned its foibles, the general accuracy of deflection shooting improved dramatically. During 1944 an analysis of 130 combats by Spitfire Mk IXs fitted with fixed-graticule sights revealed that there had been 34 kills - 26 per cent of the total. During the same period, one squadron operating the same Spitfire variant fitted with the new gunsight took part in 38 combats, scoring 19 kills - 50 per cent of the total. The new gunsight virtually doubled the effectiveness of air-to-air gunnery. With the new sight pilots reported scoring hits on evading targets at ranges as great as 600 yards, and at deflection angles of up to 50º.'
I don't think the gunsights could be defined as "next to useless", but maybe the British and American sights were different.
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We few, we happy few, we band of brothers;
For he to-day that sheds his blood with me
Shall be my brother
Bring the Spitfire F.MkXIVc to Aces High!!!
Sisu
-Karnak