Gun harmonisation

In aerial gunnery, gun harmonisation, convergence pattern, convergence zone, convergence point or boresight 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 boresighted to point straight ahead; instead they were aimed slightly inward so that the projectiles met at one or more areas several hundred yards or meters in front of the fighter's nose. The intent was to concentrate the fire of multiple weapons to deliver greater damage to the enemy. A limitation of harmonisation was that targets closer or farther away than the effective 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 light 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 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 frontline 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 Spitfire and the 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.

Distance
The distance of the convergence point depended on the ballistic performance of the projectile. Standard early war machine gun rounds such as were fired by the British .303 Browning machine guns did not travel as far as later heavy machine gun rounds or cannon shells, so the lighter rounds were focussed into a cluster or point at shorter distances. All machine gun rounds do more damage at closer distances, so a closer point was often preferred for increased damage, especially for target areas protected by steel plate, such as armoured cockpits. However, if a close point was chosen then a distant enemy might be safe from the fire of wing guns, the rounds passing ineffectually on both sides of him. The opposite situation was not so much of a problem; a distant boresight point would not usually stop a fighter from delivering damage at close range, though the hits would not be concentrated on target. Tactical decisions also dictated whether a fighter unit would choose a closer or farther away convergence point. A twisting and turning style of dogfighting might indicate a shorter distance, while energy tactics such as diving to gain a speed advantage might indicate a greater distance.

Early mark British Spitfire and Hurricane fighters firing the .303 round had their eight wing guns focussed into a convergence zone 1350 ft or 1180 ft forward during the early part of the war, as initially favoured by Air Chief Marshall Hugh Dowding, but combat experience showed that shorter distances were more effective, and the convergence distance was reduced to 750 ft or even 360 ft. Various distances that were employed in WWII by American fighters using .50 inch (12.7 mm) heavy machine gun rounds include 500 ft, 750 ft, 900 ft and 1000 ft, with the longer distances favoured later in the war.

Some pilots preferred more than one point of convergence. In 1944 operating out of England, American Lieutenant Urban "Ben" Drew set the .50 in guns of his North American P-51 Mustang "Detroit Miss" to converge at three points: 600 ft, 750 ft and 900 ft, with the inboard guns aimed closer and the outboard guns farther away. Drew felt that this gave him a suitable concentration of fire over a deeper envelope of engagement distance.

Night fighter wing guns of all belligerents were often set to converge at relatively close distances such as 450 ft for the UK. Night fighter tactics using wing guns called for a surreptitious approach on the tail of the enemy, surprising him with fire at a chosen distance.

A very close convergence point proved devastatingly effective for some pilots. The highest scoring fighter pilot in the world, German Major Erich Hartmann, set the wing guns (later cannon) of his Bf 109 to converge at 50 m because of his preference for waiting to attack until very near his opponent. In the Pacific War in mid-1943, American Marine Fighting Squadron 213 harmonised the six 0.5 in wing guns of their Mk I Vought F4U Corsairs to converge to a point 300 ft ahead. The squadron's usual tactic was to dive upon an enemy from the front and slightly to one side (a high-side attack using full deflection) and fire when at the convergence distance. American ace Major Bill Chick of the 317th Fighter Squadron based in North Africa in January 1944 bore-sighted the eight 0.5 in guns of his Republic P-47 Thunderbolt to converge at 300 ft because he did not care for deflection shots and instead attacked his targets from the rear at that distance.

Size of pattern
Early in WWII, the British 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 at 750 ft. This was referred to as the "Dowding spread" because the Air Chief Marshall advocated such a large pattern to make it more likely that a fighter pilot would obtain a hit. After evaluation in battle, by mid-1940 all British fighters were set to fire into a single point at 750 ft rather than a larger area. The British observed that too many German bombers were returning home after taking many rounds of dispersed fire. It was decided to concentrate the fire in a much tighter pattern. However, the flexibility of the wings could contribute to a larger-than-intended convergence pattern, especially with thinner wings as on the Spitfire. Any flexing of the wings in flight would cause movement of the gun mounting which would affect the aim.

Some American groups also converged their guns in a rectangle. USAAF Major James White described how the Mustangs of his 487th Fighter Squadron were harmonised to fire their six guns into a wide rectangle 10 by at 450 ft. The outer guns of the Mustang were 15.846 ft apart, so this ten-foot box narrowed in width as the firing distance increased.

Central guns
Fighters with central guns, mounted in the fuselage or a central gondola, typically aimed them straight ahead rather than converging them. A fighter such as the German Bf 109E "Emil" model carried a combination of central and wing guns; the wing guns were converged to a point but the central guns could always be counted upon to aim directly at the target.

The British Whirlwind (four 20 mm cannon) and the American P-38 Lightning (one 20 mm cannon and four 0.5 in), both twin-engine heavy fighters, carried the entirety of their gun armament in the nose, a configuration which concentrated the firepower at all distances, and did not require harmonisation.

The North American F-86 Sabre, a 1947 jet fighter-bomber design used by US forces in the Korean War, was equipped with six 0.5 in machine guns, three mounted on each side of the nose, the two sides spaced approximately 4 ft apart. These guns were harmonised to converge at 1200 ft.