QUOTE(Sadistic Reaper @ Sep 21 2010, 10:44 AM)
It all depends on the tier tree of your tank, turret and the type of ammo you use, and where you fire at a tank's location.
HE rounds deals more damage and does AOE radius (not that big) and spreads the damage in the tank but will have a lesser armor penetration. But no matter the distance of the enemy tank, HE rounds will not be affected by range. It will still cause damage to the enemy tank (again, depending on your turret). HE rounds are excellent to cause more damage towards treads and engine, causing the enemy tank to be immobilize.
AP has more penetration and deals lesser damage. It all depends on your tank's turret. AP gold ammo has more penetration with the same damage (provided if you can fork out the gold coins coz it's expensive). Also, the longer distance your target is, the lesser your Armor Penetration will be (again, depending on your turret). Also, depending on your turrent, usually a higher damaging turret will have lesser penetration, thus, you can try to calculate the damage/penetration ratio for the turret's difference in your tank's research window.
Most of the time, if your tank is from tier 3-4, you can't damage a heavy tank at all because of your light/medium turret *(again, depending on the tank's turret in the tree). However, if a heavy tank is immobilized due to damage from engine/tracks/turret/ammo, they will have an easier take to receive damage and takes a bit more damage than earlier before.
It also depends on where you fire on a tank, the sides and back of the tank has lesser armor compared to the front. Also, by using HE and firing at the back of the tank, not only it'll damage their engine but there's a percentage on causing their engine to caught on fire.
If you're finding yourself unable to cause damage to a heavy tank, aim for their tracks to immobilize them, so your other friendlies can take em out.
Also, equip your tank with both types of ammo with AP and HE as it'll help you in the battle (HE for long range, AP for short)
Hope that helps, i never tried medium tanks before coz all the while, i'm using tank destroyers but i'm aiming for a medium tank now.
I think you're referring to the Cannon/Gun instead of the Turret in the bolded areas. Different turrets mostly just affect how protected your turret is and how fast it can turn (Traverse speed).
Also, using HE rounds in general doesn't just mean you have a higher chance of damaging tracks and engines, you pretty much have a higher chance to damage every module in a tank:
Radio - Limits their ability to broadcast or receive other tank information
Gun - Lower accuracy
Turret - Lower turret traverse speed
Ammo rack - Slower reload (Assuming you haven't blown up)
Viewfinder - Shorter view range
Fuel tank - Shorter range? (I have no idea what this does really)
But general rule of thumb is this:
AP round = Against weaker/lower tier enemies, rear of tanks, SPGs
HE round = Everything else... Basically, your backup plan if AP rounds fail.
Also worth noting is that AP round penetration is actually based on the contact angle. So, if possible, try to hit the tank armour at 90 degree angles to get the maximum penetration for your shot. I've seen many players just spam the same spot on a tank and watching the shots ricochet because its so obviously a bad angle (Funny when you see KVs bouncing multiple shots off an A-20).