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. 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, usually mitigating 5 degrees of armor sloping, and lose some penetration value over distance.
Armor Piercing Composite Rigid (APCR) Shell
This type of ammunition has the highest penetration value, but the damage value is the same as an AP shell. It is subject to normalization, although it only normalizes by 2 degrees. 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 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. Nevertheless, the APCR used by most medium and high tier guns will still have more penetration than standard AP at longer ranges.
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.
High Explosive Anti-Tank (HEAT) Shell
It carries a higher penetration value than AP, but unlike AP and APCR, is not subject to normalization, and therefore 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. HEAT also has slow muzzle velocity.
High Explosive Squash Heads (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 armor, 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 Centurion 7/1(with the 105mm Royal Ordinance L7A1)and the FV4202 use HESH as their premium ammunition. It will also soon be added to the British tier 9 and 10 Heavy tanks, replacing the slot of their regular HE round.
Note: this type of ammunition is only avalable for British tanks.