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.
Background 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. This made the Supermarine Spitfire and the Hawker Hurricane the most heavily armed fighters in the world at the time, but there arose a lively debate about how these guns should be converged.
Size of pattern
Early in World War II, the British were in favour of "pattern harmonisation", a shotgun-like tactic which disperses the fire of multiple guns to gain a greater chance of a hit. The Royal Air Force (RAF) tried various patterns of gun harmonisation, with the convergence area taking the form of a rectangle or a circle. In December 1939, No. 111 Squadron RAF adjusted its Hurricanes to fire into a wide rectangle that was 12 by 8 ft (3.7 by 2.4 m) at 750 ft (230 m). This was referred to as the "Dowding spread" because Air Chief Marshal Hugh Dowding advocated such a large pattern to make it more likely that a mediocre fighter pilot would obtain a hit. The pattern converged further with distance, tightening the most at about 1,200 ft (370 m) or even 1,950 ft (590 m), the latter reported by Len Deighton. New Zealand's top ace Colin Falkland Gray expressed frustration at the recommended scheme, saying that the wide pattern and long distance penalized those pilots who were excellent shots. Gray recommended the guns be converged to a point at 750 feet. South African ace Adolph "Sailor" Malan agreed so strongly with Gray that he went ahead and set his own guns to converge this way, subsequently telling fellow airmen how much better it worked. The British observed that too many German bombers were successfully disengaging from battle after taking many rounds of dispersed fire. It was decided to test a much tighter pattern. After evaluation in battle, by mid-1940 pattern harmonisation was dropped by the RAF in favour of "point harmonisation". Following the lead of Gray and Malan, British fighters were generally set to fire into a single point at 750 ft rather than a larger area. By September 1940, better results were reported. Whatever pattern was chosen, the flexibility of the wings could contribute to a larger-than-intended convergence pattern, especially with thinner wings as on the Spitfire. The vibration of the engine, the twisting pull of the propeller, and the flexing of the wings in flight would cause movement of the gun mounting which would affect the aim. The normal vibration of the guns as they fired would also spread the shots; an intended point convergence was at best a somewhat larger grouping of shots within a circle. At a distance of 1,000 ft (300 m), the tightest practical grouping of shots would range throughout a circle about 4 ft (1.2 m) wide. Furthermore, the physical arrangement of guns in the wing had an effect on the convergence pattern. The Spitfire's guns were spaced relatively far apart in each wing, which meant that their gunfire was more dispersed before and after the range of greatest convergence. The Hurricane's guns were closely spaced, allowing more confidence that a grouping of bullets from one wing would cause heavy damage, even if the other wing's bullets missed the target. While Spitfire squadrons might converge their wing guns at a different distance for each left–right pair, to give a deeper envelope of damage, Hurricane squadrons usually aligned the guns in each wing to shoot nearly parallel, with all gunfire coming together at the same range. As a result, the Hurricane outperformed the Spitfire in delivering damage to German bombers during the Battle of Britain. Some American air units also converged their guns in a rectangle. USAAF Major James White described how the North American P-51 Mustangs of his 487th Fighter Squadron were harmonised to fire their six guns into a wide rectangle 10 by 6 ft (3.0 by 1.8 m) at 450 ft (140 m). The outer guns of the Mustang were 15.846 ft (4.830 m) apart, so this ten-foot box narrowed in width as the firing distance increased.
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