Crazy thing I found in the pacific fighters manual

[Billfish]

psst.....aircraft has skin too


The problems with many explosive "MG" sized rounds early on was that they often showed little effect, even less then standard AP, and even in some cases less then a standard round.......The trouble being, they tended to explode on impact upon the outside of the aircrafts skin (that being if they didn't before leaving the barrel), the explosion actually causing the round to shatter and blow away/back from the aircraft sometimes even resulting in zero penetration.