Durability of aircraft to MG (< 8mm) guns

[winny]

The main cause for the majority of the holes in German bombers was simple ball ammo. RAF fighters usually had 3 or 4 guns with ball ammo 1 or 2 with AP.

The problem, as someone already said, is that the bullets would deflect or start to tumble and not hit at the correct angle. Only a quarter of the armour piercing rounds fired into the Blenheim reached the armoured plate and hardly any penetrated it.

The same round fired straight at a 12mm armoured plate (8mm thicker than the Blenheims) from the same distance (180m) penetrated the armour 100% of the time. Airframe makes a huge difference.

The Germans knew that AP rounds were ineffective against bombers and they eventually removed them from the ammo belts, except for specialised missions.