I see nothing wrong with an aircraft to absorb a 30mm round or two in the fuselage.
Sure, it would punch a decent sized hole in the fuselage but that shouldn't be enough to bring an aircraft down with 1 hit.
If you want one-hit kills with the 30mm cannon, hit the cockpit, the wing tips or even the engine, that will disable most aircraft and enable you to come back around and finish it off.