This rig has always fascinated me. The link seems to show that the 20mm cannon was situated behind the engine. There must have been a stationary ‘barrel’ through the crankshaft. These figers, along with their Allied counterparts could only carry so much ammunition. 20 mm shells are heavy and bulky. The room in the cockpit suggests that the pilot ‘wore’ the airplane.
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On a tangent; Field Marshall Goering is once said to have asked of the General of the Fighters, Adolf Galland, if there was anything the general needed to better press the air war to the Allies. Galland is said to have replied, “ send me a squadron of Spitfires.”
Edit: 1) 20mm equals approx. 3/4 “, that’s a fairly stout hole through the bulkiest part of the IC engine. (2) Looking more closely at the diagram, I have to agree with Antiwar in that the 20mm cannon is situated below where the crankshaft would be in an inverted mounted V-12.
There is some debate as to the meaning of the quote. Later in his book, Galland states that he was pleased with the capabilities of the 109 vs the Spitfire and was trying to tweak, the Luftwaffe chief.