[Imp]

The situation you mention does not really occur it the unit does not select one type of ammo use it then switch to the other. Rather 99.9% of the time the crew choses the best ammo for the target switching as needed. If it runs out of one type its because thats what the target required & running out of AP in WW2 normally means a lot of hard targets as in armoured vehicles.
At long range with low penetrating AP ammo sometimes the crew will fire HE instead as it has more chance of effecting the target as its effect does not fall off with range.