It's a known issue Howi. It's more of a overall DM problems rather than just with the 30mms. The past discussions on this problem has more or less led to the conclusions that;
1) the DM in AH deals "undamaged" or "damaged" status. No in-betweens or gradual state of damage.
2) when a "damaged" status is dealt out, the 3D model is effected to correspond to the levelof damage. For instance, if a "wingtip" area gains enough damage and is declared "destroyed", the wingtip falls off from its 3D model
3) however, in case of the engine area, there is no damage here. If the engine is declared "damaged", either the engine siezes up or starts losing oil. Since there is no fuel tank near the engine, or since the 3D model doesn't allow the engine block to fall off or something, no damage obtained in this area will cause any kind of instantaneous kill or structural failure.
4) therefore, if by chance you land a 30mm shell somewhere in front of the cockpit, between the cowl and the spinner area, the plane will obtain only engine damage of some sort. In real life, a 30mm hit at that area might even totally separate the engine block from the plane, or cause an instant detonation that might blow up the plane. However, in many cases, not so in AH.