Not quite.
In fact, only a few early WW1 attempts at synchronisation worked as interrupters (although the system was often known as 'interrupter gear' for some time thereafter).
The issue of synchronisation and its effect is dealt with in detail in 'Flying Guns – World War 1: Development of Aircraft Guns, Ammunition and Installations 1914-32' by Emmanuel Gustin and myself, but to summarise:
Synchronised MGs were in fact semi-automatic; they were fired not by the pilot but by the synchroniser gear (SG) - pushing the firing button just sent a message to the SG to 'fire when ready'. The SG would then fire the gun each time a propeller blade was out of the way. In fact, guns couldn't fire fast enough to keep up with the propeller. A typical prop might spin at (say) 1,200 rpm, which means that if it has three blades there are 3,600 opportunities per minute to fire one shot in each gap between blades. So in reality guns only fired one shot - or less - for every rotation of the prop.
Fixed props as used in some early WW2 planes meant that prop revs varied, so this affected the guns' RoF. Finnish pilots complained that on some of their aircraft (e.g. Fiat G.50 fighters) the guns' RoF slowed right down at some engine revs. This extract from 'FG: WW1' explains why:
"The effect of synchronisation on the rate of fire can best be explained by describing a simple system like that introduced by Fokker, in which one firing signal was sent to the gun for each rotation of the propeller. If the gun was capable of firing at 500 rounds per minute, then for propeller speeds of up to 500 revolutions per minute the RoF would be the same as the propeller rpm. However, as soon as the propeller exceeded 500 rpm, the gun mechanism could no longer keep up and could then only fire on every other rotation, so the RoF would drop to 250 rpm. It would then accelerate again with increasing propeller speed but at half the rate, so when the propeller was spinning at 1,000 rpm, the gun would be back to firing at 500 rpm again. Once more, propeller revs faster than this would cause the RoF to drop, but this time only to two-thirds of the full RoF, as it would fire on every third rotation, so it would be achieving 330 rpm. As the propeller continued to accelerate to 1,500 rpm, the gun would be back up to 500 rpm again, and so on. Any quoted figure for synchronised rates of fire could therefore only be an average."
The introduction of variable-pitch props in WW2 largely solved this problem, in that the prop speed was kept constant. Even so, the loss of RoF with synch guns depended on the system and on the gun. One of the worst was the Browning which in US and Japanese installations lost as much as 40% of its RoF. The best was the German electrical synch system which kept the losses down to about 10%.
Tony Williams: Military gun and ammunition
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