Since a hot debate is going on on another forum which intersects with this .303 thread I thought I'd share this quote with you (the 'myself' mentioned is Anthony Williams, Co-author of 'flying guns' which I can highly recommend);
"On the contrary, rifle-calibre bullets were already proving themselves to be inadequate in dealing with the new, tougher structures required for the fast monoplanes - even without armour. The extract below, from 'Flying Guns – World War 2: Development of Aircraft Guns, Ammunition and Installations 1933-45' by Emmanuel and myself, describes the results of British tests of .303 and 7.92mm armour-piercing ammunition on a Blenheim - scarcely the most strongly-built of aircraft:
"The test then changed to shooting at the rear of the long-suffering Bristol Blenheim at the same distance [200 yards], involving penetrating the rear fuselage before reaching the 4 mm armour plate protecting the rear gunner, which was angled at 60º to the line of fire. The results in this case were reversed; 33% of the .303" rounds reached the armour and 6% penetrated it. In contrast, only 23% of the 7.92 mm bullets reached the armour, and just 1% penetrated. The British speculated that the degree of stability of the bullets (determined by the bullet design and the gun's rifling) might have accounted for these differences."
Please note that only a third of the British AP bullets - and less than a quarter of the German ones - reached the armour at all; the rest were deflected or absorbed by the structure. That no doubt accounts for the German bombers which reached base despite being riddled with .303-shaped holes.
It's interesting just how much the penetration was reduced by passage through the light alloy of the Blenhiems fuselage. A move to the 20mm Hispano was definitely a Good Thing!