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.

[Skoshi Tiger]

It really depends on what you term 'Bounce'

You can bounce a 50 cal bullet (or any projectile for that matter, stones shells etc. ) off the surface of water if you get the angle right, and (in a classical monty python leap of logic!) the human body is 70% water. So given the right angle!