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... had the Whirlwind gone into mass production deploymnent, what under-served niche would it have fulfilled?
The Merlin Whirlwind was proposed by Petter, by-passing the normal chain of command, directly to Sholto-Douglas. It was a last ditch effort and there is no evidence that Westland had really solved the problems of fitting the Merlin in the aircraft, particularly the extensive redesign of the nacelles and landing gear attachments that would be needed to accommodate the Merlin's up draught carburettor.
The point about the engines is important. When the first Peregrine engine was delivered the decision had already been taken to cease production after just 290 units. From that point the Whirlwind was doomed, it was just a question of how many, if any, Whirlwinds would be produced. The eventual 114 is about as many as was ever going to be possible.
I'm not sure about the carnage either. Yes, the Whirlwind was armed with four 20mm cannon, but with just 60 rounds for each, that's roughly two three second bursts. The armament was not as reliable as the pilots would have hoped either.
I would note that ALL the 109s in the BoB had 60 round drums.
Various armament combinations were suggested for the Whirlwind, but the four drum fed cannon arrangement that went into production did not have any kind of selector switch.
Rate and weight of fire were considered vital to the chances of shooting down an enemy in the fleeting time expected to be available. All four cannons fired simultaneously (when they worked). The cannons were fired by a single button on the spade grip of the control column.
The cannons, cocked on the ground, were fired hydraulically (up to P6969) and pneumatically on all subsequent aircraft. A field modification would probably have been beyond the squadron capability and would have required approval at a higher level anyway.
Cheers
Steve