I have some information about the 75mm gun on the Mitchell but they could only fire four rounds at a time as the recoil would slow the aircraft down so much that it could stall.
Err, no…this is a quote from Flying Guns – World War 2: Development of Aircraft Guns, Ammunition and Installations 1933-45 by Emmanuel Gustin and myself:
This might be an appropriate moment to dispel one of the favoured myths of big-gun aircraft; that the recoil had a drastic effect on their speed. To take the example of the USAAF’s B-25 fitted with a 75 mm M4 gun; the aircraft weighed around 12,000 kg and attacked at perhaps 400 km/h, the gun fired a 6.8 kg projectile at around 2,200 km/h. A simple rule of thumb is to multiply the weight by the speed to achieve a rough “momentum index” (it is actually a bit more complicated than this, as the expanding propellant gasses contribute to the recoil). It will be apparent that the aircraft has at least 200 times the momentum of the projectile, and a single shot will therefore not greatly slow it. In fact, at the end of an attack run in which several shots were fired, the plane would typically be slowed by 10-15 mph.
Four rounds was about as many as could be fired in one attack run, as the gun was manually loaded.
The 75mm Pak 40 fitted to some Ju 88 and (as the BK 7,5) to some Hs 129, was a more powerful gun than the 75mm fitted to the B-25, and had a significantly longer cartridge case.
Tony Williams: Military gun and ammunition website and discussion forum
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