Obsolescence
The usefulness of synchronization gears naturally disappeared altogether when jet engines eliminated the propeller, at least in fighter aircraft; but gun synchronization, even in single piston engined aircraft, had already been in decline for the decade prior to this.
The increased speeds of the new monoplanes of the late 1930s meant that the time available to deliver a sufficient weight of fire to bring down an enemy was greatly reduced, and two guns were no longer enough. Since it was never practicable to mount more than two synchronized guns in the forward fuselage, any additional guns had to be mounted in the wings. Cantilever monoplane wings provided much more space than the fuselage to mount armament — and being much more rigid than the old cable braced wings they provided almost as steady a mounting as the fuselage. The retention of fuselage mounted guns, with the additional weight of their synchronization gear (which slowed their rate of fire, albeit only slightly, and still occasionally failed, resulting in damage to propellers) became increasingly unattractive as the extra firepower offered by two machine guns came to represent a decreasing percentage of a fighter's total armament.
Against these arguments, "centralised" guns still rewarded the true marksman, regardless of advances in gunsight technology: their range was limited only by ballistics, as they did not need the "harmonisation" necessary to concentrate the fire of wing mounted guns. These considerations resulted in a reluctance (especially in Germany, Russia, and Japan) to abandon fuselage-mounted guns altogether—in fact, even the old (and highly problematic) idea of mounting a cannon to fire through the centre of the propeller hub for geared-propshaft, inline-engined fighters, pioneered by the World War I-era SPAD S.XII's 37mm calibre moteur-canon, was revived to this same end, notably in the mass-produced Messerschmitt Bf 109 in its Motorkanone mount.
The very last synchronizer-equipped aircraft to see combat action were in fact the Lavochkin La-11 and the Yakovlev Yak-9 during the Korean War.
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