proven effectiveness of cannon shells on planes

over the years its undeniable that there has been a lot of air to air combat with jet fighters of various kinds, types, capabilities and with various weapons. many kills was scored with the cannon, getting in close and shooting to bring down the other plane. guns ranged from 20 to 23 to 30 mm. shells would be armour piercing, tracer and high explosive(incendiary?) ammo and the effects on a plane can be seen to be deadly with the right amount of hits in the right place.
does anyone have any photos of planes that got back to base with cannon shell damage, with various types of ammunition eg 20 mm high explosive? and to compare to result of 20 to 30 mm damage? there must be some studies done on the ballistics and impact damage of this subject matter? what do you all think?