Editing Synchronization gear (section)

=== At the gun ===
[[File:Synchronised gun - unsuitable gun or amunition.svg|thumb|left|180px|An attempt to synchronise an unsuitable gun or faulty/disparate ammunition – "rogue" shots – some of which risk striking the propeller.]]
The second requirement was for a gun that would reliably fire (or hold its fire) exactly when required. Not all automatic weapons were equally amenable to synchronization. When it was ready to fire, a synchronized machine-gun needed to have a round in the breech, the breech closed, and the action cocked (the so-called "[[closed bolt]]" position).<ref name=Volker3p52>Volker 1992, pt. 3, p. 52</ref> Several widely used automatic weapons (notably the [[Lewis gun]] and the Italian [[Fiat–Revelli Modello 1914|Revelli]]) were triggered from an [[open bolt]], with an unpredictable interval between triggering and firing,<ref name=Williams1.34>Williams 2003, p. 34.</ref> and were thus not suitable for synchronization without extensive modification.<ref name=Woodman4a>Woodman 1989, pp. 176–177.</ref>

In practice it was found necessary for the gun to be fired in [[semi-automatic firearm|semi-automatic]] mode.<ref name=Volker2p79>Volker 1992, pt. 2, p. 79</ref> As the propeller revolved, a series of firing impulses was transmitted to the gun, each of which could trigger it to fire a single shot. The majority of these impulses would catch the gun in the process of ejecting a spent round or loading a fresh one, and would thus have no effect; but as soon as the firing cycle was completed, the gun would be ready to fire as soon as it received the next impulse from the synchronizing gear. The delay between the end of the firing cycle and the arrival of the next firing impulse slowed the rate of fire in comparison with a free-firing machine gun, which fires the moment it is ready to do so; but provided the gear functioned correctly, the gun could fire fairly rapidly between the whirling propeller blades without striking them.<ref name=Volker2p78>Volker 1992, pt. 2, p. 78</ref>

Some other machine-guns, such as the Austrian [[Schwarzlose MG M.07/12|Schwarzlose]] and the American [[M1895 Colt-Browning machine gun#World War I|Marlin]], proved less than perfectly adapted to synchronization, although eventually predictable "single shot" firing was achieved, typically by modifying the trigger mechanism to emulate "closed bolt" firing. Most weapons that were successfully synchronized (at least in the First World War period) were (like the German [[Parabellum MG 14/17|Parabellum]] and [[Maschinengewehr 08|Spandau]] guns and the British [[Vickers machine gun|Vickers]]) based on the original [[Maxim gun]] of 1884, a closed bolt weapon operated by barrel recoil.<ref name=Williams1.16>Williams 2003, pp. 16–17.</ref> Before these distinctions were fully understood, much time was wasted on attempts to synchronize unsuitable weapons.<ref name=Volker1p48/>

Even a closed bolt weapon needed reliable ammunition.<ref name=BAP11>Bureau of Aircraft Production 1918, p. 11.</ref> If the [[Primer (firearms)|primer]] in a cartridge is faulty to the extent of delaying the firing of the gun for a tiny fraction of a second (quite a common case in practice with mass-produced ammunition) this is of little consequence in the case of a gun in use by infantry on the ground, but in the case of a synchronized "aircraft" gun such a delay can produce a rogue firing, sufficiently "out of time" for it to risk hitting the propeller.<ref name=Williams1.35>Williams 2003, p. 35.</ref> A very similar problem could arise where the mass of a special round (such as an incendiary or explosive one) was different enough to produce a substantial difference in muzzle velocity.<ref name=Robertson105>Robertson 1970, p.105</ref> This was compounded by the additional risk to the integrity of the propeller due to the nature of the round.

The "trigger motor" could theoretically take two forms. The earliest patent (Schneider 1913) assumed that the synchronization gear would periodically ''prevent the gun from firing'', thus operating as a true, or literal "interrupter". In practice all "real-life" synchronization gears, for which we have reliable technical details, directly ''fired the gun'': operating it as if it were a semi-automatic weapon rather than a completely automatic one.