Gun harmonisation

In aerial gunnery, gun harmonisation, convergence pattern, convergence zone, convergence point or bore-sight point refers to the aiming of fixed guns or cannon carried in the wings of a fighter aircraft. The wing guns in fighters were typically not bore-sighted to point straight ahead; instead they were aimed slightly inward so that the projectiles met at one or more areas several hundred yards or metres in front of the fighter's nose. The intent was either to spread the fire of multiple weapons to increase the chance of a hit, called "pattern harmonisation", or to concentrate the fire to deliver greater damage at one point, called "point harmonisation". A drawback of harmonisation was that guns worked effectively in a limited zone, so targets closer or farther away from the zone were not damaged as much, or were completely missed. The rounds would diverge further apart after passing through the convergence point. The convergence of multiple guns was a common practice from the 1930s to the 1950s, especially in World War II. Military aircraft from the 1960s onward generally did not carry guns in the wings, so convergence was not as much of a concern. As World War I came to a close, the standard pursuit (fighter) armament was two rifle-calibre machine guns mounted on the cowl of the fuselage, synchronised to fire between the propeller blades, a process which slowed the rate of fire. In the late 1920s and 1930s, electrically controlled firing mechanisms, (coupled with the advent of more reliable weapons systems that did not require a pilot close by to clear any malfunctions), allowed aircraft designers to place guns in the wings, negating the need for synchronisation. These wing guns could fire at their maximum rate; they were aimed slightly inward to give a converging field of fire at a certain range. In the mid-1930s when the front-line fighters of many countries including Italy, Japan, and the US were still using only two synchronised guns on the fuselage, the UK ordered their fighters to carry eight guns, four in each wing.