Ammo
The amount of ammunition of each tank can carry depends on the gun calibre 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.
Shell Types
Armour-Piercing (AP) Shell
This shell does the full damage listed on the gun specifications if it penetrates the enemy's armour. 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.
Armour-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 calibre guns such as a 57mm gun will lose penetration at a much faster rate than high calibre guns such as a 120mm gun. At the very high end of tank gun calibres, 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 armour and the shell's penetration value increases. If it manages to penetrate the armour, 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 therefore must penetrate the effective armour value at the given impact angle, meaning HEAT will have trouble with very thick, sloped armour more than AP and APCR. However, also unlike AP and APCR, HEAT does not lose penetration value over distance.
High Explosive Squash Head (HESH) Shell
This shell functions as high explosive ammunition with very high penetration, and behaves the same way -- if it does not penetrate, it will explode on the surface of the armour, dealing marginal damage. It also deals additional damage to modules and crew.
HESH replaces the standard HE ammunition on several high-tier British tanks. The FV215b (183) and the FV4202 use HESH as their premium ammunition.