The simplest type of gunsight is fitted to small arms such as HMGs intended for ground fighting, in which there is time to aim and targets do not usually move quickly. All that is required is an aperture, adjustable to allow for range, at the back of the gun and a reference point such as a bead or blade on the barrel, which is lined up with the target. Tracer ammunition can be used to correct the aim. Telescopic sights are used to improve long-range accuracy, particularly in vehicle mountings, and image-intensifying or infra-red sights are increasingly common to assist night fighting.

Range-adjust able rearsight on a .50" Browning. Note the spade grips and central trigger (Courtesy: MoD Pattern Room)

Aircraft reflector sight, British, WW2 (Courtesy: R.W.Clarke)

use of the reflector sight, in which the aiming marks were projected onto an angled glass screen, arranged so that they were in focus at infinity so that the gunner did not have to refocus between the aiming mark and the target. First fitted to fighter aircraft, these were later applied to other uses.

A further advance in aircraft gunsights during the war was the gyro sight, in which an integral gyroscope was used to assist deflection shooting (i.e. aiming in front of a manoeuvring aircraft in order to hit it). This worked by resisting the movement of the aiming mark as the aircraft or turret was swung to keep the target in the sight. The fighter pilot or gunner had to swing the plane or turret to a greater degree in order to get the aiming mark onto the target, thereby automatically allowing for deflection. This was an important advance, especially for fighter aircraft. German analysis in the early 1940s showed that the maximum deflection angle for effective shooting without a gyro sight was 15°, with most successful attacks being at zero degrees deflection. However, German gyro gun-sights were not as good as the Allied ones.

Radar-directed fire control systems for naval AA weapons were introduced during the Second World War, although nearly all automatic weapons remained manually aimed. Radar is now an

Oerlikon Gai D01

Optical sight for Oerlikon 20mm GAI-D01 A A gun

(Courtesy: Oerlikon-Contraves)

Optical sight for Oerlikon 20mm GAI-D01 A A gun

(Courtesy: Oerlikon-Contraves)

essential component of gun-type naval CIWS (close-in weapon systems) and army AA gun systems as well as larger naval guns. Radar gunsights for aircraft were also under development during the war but these did not reach service until later, and only became dependable much later still.

The more sophisticated modern optical AA gunsights include a laser rangefinder linked to a ballistic computer to create a lead computing sight, able to calculate the distance, direction and speed of the target and adjust the sights accordingly. They can also allow for any tilting of the gun mounting, and in SPAAGs for the movement of the vehicle. Such facilities are also common in main battle tanks and are beginning to appear in MICVs (mechanised infantry combat vehicles). Also becoming common are optronics systems (from 'optical' and 'electronics1; basically, video cameras incorporating night vision capability) which are often used as a back-up to radar-controlled AA mountings in the most modern naval and SPAAG systems.

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