You need to be careful about the difference between 'maximum' and 'effective' ceiling of AA guns. Maximum just means - how high will the shell get if the gun is fired when pointing straight upwards? Effective means - what is the maximum altitude that an aircraft can be engaged by this gun with some chance of success?
So for the German 37mm Flak, the maximum altitude was 4,800m (15,750 feet) but the effective ceiling only 2,000m (6,500 feet). For the British 40mm Bofors, the figures were: ballistic maximum 7,200m (23,600 feet), maximum before self-destruct fuze detonates 5,000m (16,500 feet), but effective ceiling 1,500m (5,000 feet).
The definition of 'effective ceiling' is determined partly by the ballistic performance of the gun, partly by the quality of the fire control system, and partly by the criteria chosen - what kind of hit probability would you describe as 'effective'? Figures from different sources may therefore not be comparable as different criteria may have been used.
The 40mm Bofors was the most powerful of all of the automatic AA guns used in WW2 (except for a handful of German 5cm Flak 41). The Russian 37mm was marginally more powerful than the German equivalent.
Tony Williams: Military gun and ammunition
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