There was a major unforeseen problem with the .303 AP round. When it was tested, it could go through armour plate with no problem. However, if it struck another surface before it hit the armour plate (such as aircraft skin), the energy would dissipate quickly and not have enough energy to penetrate or even reach the armour.
http://www.quarry.nildram.co.uk/BoB.htm
I'm sure I've got some more details of the tests somewhere.