Durability of aircraft to MG (< 8mm) guns

[major_setback]

Originally Posted by winny

This might be of interest to you.

Tests by the RAF indicated that both the .303 and 7.92mm AP bullets had some problems penetrating the structure of the relatively small and light 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. The results were poor; 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.

Notice how close they were fired from, and the He-111s had better armour than a blenheim.
Add to that the fact that only 2 of the 8 guns would have had AP rounds and you end up with a lot of German bombers getting home when they shouldn't have.

This was not the case when the roles were reversed.

Yes, there was one pilot, can't remember who (Moran?) who purposely didn't finish off bombers, and let them return damaged because it was a bigger blow to morale if the rest of the bomber unit saw a bomber with dead crew members return, than if it just went missing.
The bomber would be out of action anyway until repaired.