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Bullets hitting the aircraft skin at shallow also deflects a lot more rounds than most people realize, I'd bet.
The extension plate and ammunition box armour were not fitted in 1940. Maybe my 'internal armour', Dr Price's description, not mine, includes the windscreen which did become internally fitted ?
Cheers
Steve
This could also include the fireproof bulkhead, which I've seen many sources include in the 'armour' category for the Hurricane and Spitfire.
Tests conducted at Orfordness indicated that both .303 and 7.92mm AP bullets had some problems penetrating the structure of a Blenheim bomber. Both guns were fired at a range of 200 yards (180m) through the rear fuselage at the 4 mm armour plate protecting the rear gunner, which was angled at 60º to the line of fire. Only 33% of the .303" rounds reached the armour (the rest being deflected or absorbed by the structure) and 6% penetrated it. In contrast, only 23% of the 7.92 mm bullets reached the armour, and just 1% penetrated.