Air units-> no use armorpiercing ammo.

[Imp]

Mi-24P it Weapon # 153 -- GH-30K

If you look at that weapons stats

HE & AP have a Pen of 6 with range 4Km
APCR has a PEN of 8 with range 2Km

This means the APCR is only more effective than the HE at ranges below about 1.5Km maybe a bit less.

I am guessing you are firing at over 30 hexes when the HE has more chance of a kill which is why its using it.
If you want to use your AP ammo try firing at something around the 1KM mark but if its only standard AP not APCR it will still use the HE first as its better.
The HE Pen may well be an entry error being as I think its an autocannon so may need changing to 1 or 2.

Just done yourself out of some vehicle killing ammo