Before you get into combat, you should have your tank fully loaded with ammunition; if your tank is loaded with too few rounds for the match, a warning will pop up. Be careful not to click past it. The amount of ammunition of each tank can carry depends on the gun caliber and the turret, which is listed on each tank's page. The ammunition you choose will affect your combat performance. Below are brief descriptions of each shell type in the game. As of patch 0.8.1, premium ammo can optionally be purchased for credits at the standard gold-to-credit conversion rate of 1:400. In other words, a shell that costs 10 gold will cost 4000 credits.
Armor Piercing (AP) Shell
This shell does the full damage listed on the gun specifications if it penetrates the enemy's armor. However, if the shell doesn't penetrate the target, it will bounce and do no damage whatsoever. It can also damage tank modules and crew if it hits at the right spot. AP shells are subject to normalization and lose some penetration value over distance.
AP shells normally cost credits; however there is one AP round type, used by Soviet artillery armed with the 152 mm BR-2 gun, that must be purchased with gold.
Armor Piercing Composite Rigid (APCR) Shell
This type of ammunition has the highest penetration value (thanks to being subject to normalization), but the damage value is the same as an AP shell. It still has the AP shell characteristics, with slight twists. APCR has a higher muzzle velocity than AP, meaning a shell leaves your gun at a higher speed (resulting in less "leading" of distant moving targets), however APCR also loses much more penetration value over the same distance as AP. Generally speaking, low caliber guns such as a 57mm gun will lose penetration at a much faster rate than high caliber guns such as a 120mm gun. At the very high end of tank gun calibers, the penetration lost over distance is so minute that at the same range as AP, APCR will still have, or at least have a similar, penetration value as AP.
APCR shells normally cost gold. However, 7.5 made APCR shells the standard ammunition used by the new tier 10 medium tanks. Additionally, 8.2 made APCR shells the standard ammunition used by the 76 mm Gun T185 cannon, which can be mounted on the T71 American light tank, and the T69 American medium tank. They cannot use AP ammo at all, and instead can purchase this ammo using credits rather than gold. Tanks whose guns default to APCR are also not subject to the same penetration loss over distance as premium APCR, but still have the higher muzzle velocity.
High Explosive (HE) Shell
This shell has the highest damage value among all the shell types in the game, but also has a very low penetration value. These rounds have a very small splash damage area. It doesn't need to penetrate to damage the target, but the damage drops off quickly as the difference between the target's armor and the shell's penetration value increases. If it manages to penetrate the armor, it will explode inside the tank for the full damage listed in the specifications, plus HE deals additional damage to multiple internal modules and/or crew, depending on how high the HE damage value is. HE shells typically have a maximum damage value 25% higher than AP shells, rounded to the first significant figure.
Premium High Explosive (HE) Shell
Found in some SPG guns, it has no significant difference to its HE counterpart other than a larger splash-damage radius, which can be very useful for hitting tanks just behind cover that can not be hit directly. Rounds with this icon cost gold or the equivalent in silver.
High Explosive Anti-Tank (HEAT) Shell
This is the Premium ammunition for most SPG-class, tank howitzer, and many high tier tank guns. It carries a higher penetration value than AP, but unlike AP and APCR, is not subject to normalization, and therefor must penetrate the effective armor value at the given impact angle, meaning HEAT will have trouble with very thick, sloped armor more than AP and APCR. However, also unlike AP and APCR, HEAT does not lose penetration value over distance.
FAQ: When should I use AP or HE?
The general rule of thumb is; use AP on your own tier and below and HE for tanks higher than you, but this depends upon your gun: some are designed to use HE. Guns like the KV-2's 152 mm M-10 gun use a large caliber round, but have a short barrel length. The large caliber of the shell is substantial and carries a generous amount of explosive-compound within, thus causing more damage. The drawback is the shell leaving the gun will have a relatively low velocity because of the short barrel length and thus, low armor-piercing capability. When you factor in the poor accuracy and range, it's simply a waste to use AP shells with this type of gun and HE should be used exclusively. It is also highly recommended to use HE when attempting to reset a cap. You do not need to be as accurate with HE as a hit anywhere on an enemy tank will most likely cause even a slight amount of damage, and any damage, even if it's only a single hitpoint's worth, is enough to reset a tank's acquired cap points to 0. Even a near miss with HE could cause some module damage via splash. Since 8.1, gold ammo can now be bought with credits/silver. In most cases, APCR and HEAT will be much more effective at dealing with higher tier tanks than HE, which will most likely only deal very minor damage and serve only to tickle higher tier tanks with thick armor. However with APCR and HEAT, you will most likely be able to abuse weakspots your gun would otherwise need extreme angles to penetrate.
Try experimenting with different ammunition to see what your gun's capabilities are on certain tanks.When mounting tier 6 guns and higher, you should consider using AP and APCR/HEAT more frequently, as these guns have enough penetration power to damage most tanks if you know which tank and what part of the tank you should aim for.