Taking a wild guess here...
If you're catching fire from .303s could it be because the % chance of "breaking a critical component" (i.e fuel leak, oil hit, catching fire, etc.) might have something to do with the volume of fire you are receiving? For example with these made up numbers
20mm = X DMG + 20% chance to cause part failure
.303 = far less X DMG and a 5% chance to cause part failure.
A small burst of .303 might have your plane receive many many many hits due to volume of fire, yet the % chance to destroy a component might be higher, even though the X DMG to your airframe might be less
Once again just, a guess. I really wish HTC would explain the dmg modeling in more detail, but then again it I think I understand why they have not.