It is important to bear in mind that I am not talking about theoretical considerations but actual test results. Passing through a 3mm alloy skin at a sharp angle was quite enough to destabilise many projectiles.
The Germans tested their own ammunition exhaustively and the results have been preserved in penetration graphs at different striking angles and distances, with and without the intervening dural skin. Here's another quote from my next book, using information taken from this German source:
"The most exhaustive tests during the Second World War seem to have been carried out in Germany. Official penetration curves for 7.92 mm AP rounds tested in 1942 are worth examining in detail because they reveal the variations which can occur. The SmK-v (Spitzgeschoss mit Stahlkern verbessert; improved pointed bullet with steel core) achieved the following:
at 100 m: 12mm/90o, 8mm/60o, 3.5mm/30o;
at 300 m: 9mm/90o, 6mm/60o, 3mm/30o;
at 600 m: 6mm/90o, 3.5mm/60o, 1.5mm/30o;
However, if the bullet first penetrated a 3 mm dural (light alloy) aircraft skin angled at 70o, armour penetration at 100 m dropped to 4mm/90o, 3mm/60o; and 2.5mm/30o.
A different 7.92 mm AP loading, the Pz-v (Panzerdurchschlagsleitung verbessert; improved armour penetration) penetrated less than 11mm/100m/90o; but was much more tolerant of unfavourable circumstances, achieving 9mm/60o and 4.5mm/30o. Even more significantly, after penetrating the same angled dural skin the Pz-v could still penetrate 8.5 mm/90o, 7mm/60o and 3mm/30o. Clearly, bullet design made a huge difference."
I have similar stats for 13mm, 15mm and 20mm AP ammo, all showing considerable variation in the extent to which different projectiles were affected both by striking at different angles, and by penetrating a dural sheet first. The design of the projectile and (I presume) the relationship of this to the rifling twist, were key factors.
These results are facts, not opinion!