Santa, your missing the point.

Its not just the hispano that is that way. Its ALL guns.
And its not a "either or" situation. In real life if you mix a belt of ammunition, you get a mix of effects. In fact, from my reading, it was extremely common to mix ammunition types in a belt.
So if you load a belt to be AP, HE, AP, HE.
You could concievably go out and shoot down a bomber, then hit the deck and shoot up some light tanks, and then shoot down a enemy fighter on the way home.
So yes you can do both.
But realize that this isn't something that just applies to the Hispano's. Its the same with the MG151/20, the Type 99, the ShVak, or any of the others.
Now if your question is "How does Pyro calculate which variables to assign (rate of fire, muzzle velocity, weight of projectile, etc. etc.) to the weapons to obtain the effect of a mixed belt of ammunition?" Well... I honestly don't know

Ask him.
PS: If you think that only HE rounds from a cannon like the Hispano (or any other for that matter) would be effective in an air to air engagement, your mistaken. From a dead six attack, an AP round is potentially more dangerous than a HE round, because it can enter the tail of the aircraft and then continue thru the entire plane (destroying components as it goes) until it is stopped by your engine block (killing the engine).
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Vermillion
**MOL**, Men of Leisure