@Anon_Kat
AP is Armor Piercing - It does more damage against vehicles than HEAT but it has no splash damage. It does less damage to structures than HE or HEAT. If you direct hit infantry, you can one shot them 9 times out of 10. Generally, it has a faster projectile speed than an HE shell.
HE is High Explosive - It has great splash damage and is good against infantry and structures. If you land a shot on a tank with HE, you can still do some good damage but it'll be less accurate than an AP shell from a Main Battle Tank or a Tank Destroyer. HE shells are generally slow moving projectiles that have an arc.
HEAT is High Explosive Anti-Tank - It does good against everything, a jack of all trades, master of none. Good against Vehicles, Infantry, Structures, and, hope and dreams. Depending on the vehicle, it can have a drop over range. It does not have as much splash damage as HE but still very useful. It doesn't move as fast as AP, but its not as slow as HE. The projectile speed also seems to vary between vehicles using HEAT.
APCR is Armor Piercing Composite Rigid - It does more damage against vehicles than AP but it has no splash damage and does very little to structures. The downside is an inaccuracy penalty, a reload rate penalty, and range reduction. Its recommended to use it in extreme close ranges and aim for the weakspots of a vehicle.
HESH is High Explosive Squash Head - It does a good amount of damage against vehicles, has splash damage, but has a more significant drop over range, and takes longer to reload. I'm unsure if it has an inaccuracy penalty. I recommend using it if a vehicle is at almost point blank range.
The way I remember it is, if it has High Explosive in its name, its good against buildings, Infantry, and has splash. If it has Armor Piercing, its good against vehicles and infantry, but only direct hits as it has no splash.
HEAT is an all-rounder, good against everything shell.
Other ammunition types include
HEI - High Explosive Incendiary: This deals damage against infantry and can cause burn damage.
APFSDS-T - Armor Piercing Fin Stabilized Discarding Sabot: I think this type only appears on the Stryker MGS. Not the AGM one. Its acts like a hitscan AP shell for the Stryker.
ATGMs, TOWs, and other missiles - Really good against pretty much everything.
I'll update this when I hop back into the game and check the vehicles for more ammunition types. If anyone would like to make a correction to what I've written above, feel free to contact me.