| I think that JoeB has it right: the effect of recoil on speed is greater subjectively than in actuality. If I'm cruising at high speed in my car and take my foot off the gas, the sudden deceleration is noticeable. But if I look at the speedo, it shows only a drop of a couple of mph or so.
Harmonisation is a complicated subject, practices varied between air forces, at different times within the same air force, and even according to individual preference. I spent some time in the National Archives researching this, looking at contemporary documents on RAF gunnery. When the Spitfire was first regularly equipped with cannon, the RAF wanted to reduce the convergence to 600 feet, but the gun bays were too narrow to permit the guns to be angled in that much, so they had to settle for around 900 feet.
I think that convergence distances could certainly have been further for ground attack; targets on the ground were much easier to hit at long range than ones moving about in three dimensions. The Luftwaffe also used extended gun zeroing distances for use against bombers, which were big targets - I have seen figures of up to 600m (2,000 feet) mentioned.
I note that in the RAAF diagram above, the two pairs of guns converge at different distances: 250 yards and 400 yards (750 and 1,200 feet).
__________________ Tony Williams: Military gun and ammunition website | 
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