The two forms of ammunition are High Explosive (HE) and Armor Piercing (AP), denoted by the yellow and red striped shells, respectively, at the bottom of the UI. Ammo types can be switched by single clicking either "1" or "2" (by default) — in which case the selected shell will load after firing the currently loaded type — or by double-clicking the corresponding button, which will force the reload cycle with the selected type. The commander skill Expert Loader allows the force reload of the selected type to be reduced by 50% of the regular time, provided the guns are already loaded.
Armor piercing (AP) shells rely on their kinetic energy alone to penetrate the armor. This makes the AP penetration model rather complicated, in contrast to the straightforward formula employed for high explosive (HE). The shell's kinetic energy depends on its impact velocity and mass. The impact velocity in turn not only depends on the initial (or muzzle) velocity but also on the target distance: shells lose speed in flight due to air drag (wind resistance), so AP shells have the less penetration capability the further they travel. This effect varies in strength for different calibers and different nations' shells. For example, the shells used by German heavy cruiser Yorck bleed off a lot of speed on the way, making them ineffectual above about half the maximum range. Additionally, AP penetration also depends on the angle of impact.
All of this means that armor piercing shells offer high damage potential, but are highly situational and require careful aiming and knowledge of enemy ship armor schemes to use consistently. When firing AP at a target in the game, four things can happen:
- The shell strikes heavily angled armor and ricochets.
- The shell hits un-angled armor, but still fails to penetrate and shatters.
- The shell over penetrates the enemy ship by striking armor that is too thin to arm the fuse and deals 10% of its listed damage.
- The shell hits armor thick enough to arm the fuse and penetrates, doing 33% of the listed damage, or 100% if it penetrates the citadel.
It is entirely possible to have all four happen individually in one salvo (to different shells). There is also the peculiarity of a single shell doing all three forms of damage (10%, 33% and 100%) simultaneously. It is not entirely clear if this is intentional game design or not, and is difficult to reproduce.
Simply put, AP shells do more damage, but require careful aiming to achieve penetrations, while HE shells do less damage, but in turn light fires and ignore angling.
When to Use AP
Battleships should be firing AP at most targets, as their large caliber and high penetration allows them to usually do damage upon successful hits. Aiming for the waterline on most targets should be fine, but some heavily armored battleships have belt armor thick enough to bounce other battleship AP shells, so aiming above the main belt will work better.
Cruisers should fire AP when presented with an almost completely broadside target. Cruiser guns have significantly lower penetration than a battleship guns, so more careful aiming is required to hit lightly armored sections of armor. A good rule of thumb is to aim for the armor belt just below the deck line on most battleships, and at the waterline for other cruisers and aircraft carriers at close range.
Destroyers do not normally fire AP, but if presented with a perfect broadside at close range, regardless of the target's class, they should load AP. Even the 100mm guns of Akizuki are capable of landing a couple thousand damage AP salvos on a broadside battleship (particularly the superstructure). Destroyers can also use AP against other broadside destroyers, but if there is a lack of damage HE is probably the better choice.
High explosive rounds, like AP, require penetration to do damage. However, unlike AP, HE ignores armor angling and cannot over-penetrate a target (although it can still citadel). High explosive rounds will do 0 damage on shatters, 33% damage on penetrations, and 100% damage on citadel penetrations. HE penetration can be calculated for most ships by dividing the shell diameter by 6 and rounding to the nearest whole number. The HE shell can penetrate armor up to, but not including, the calculated value.
- The Tier VIII Japanese cruiser Mogami has a stock gun diameter of 155 mm. Using the Shell Diameter/6 rule, we get a value of 25.83 mm, or 26 mm. This would allow Mogami’s HE shells to penetrate armor sections up to, but not including, 26mm at any angle.
- The 203 mm guns found on Tier VIII German cruiser Admiral Hipper have an HE penetration of 203/4 = 50.75mm = 51mm.
Inertia Fuse for High Explosive
HE penetration can be increased through the use of the Level 4 commander skill Inertia Fuse for High Explosive, which increases the penetration by 30%. This is useful for allowing light cruisers with 150-180mm guns to achieve over 32mm of penetration with their HE rounds, allowing them to damage the bow, stern, and plating of many high tier battleships more consistently. It is also useful for the Tier VIII Japanese destroyer Akizuki, which has 100 mm guns, allowing her to damage other equal and higher tier destroyers with 19mm armor plating.
Using our two previous examples and multiplying the penetration by 1.3:
- Mogami: 25.83 * 1.3 = 33.58 = 34mm
- Admiral Hipper: 50.75 * 1.3 = 65.975 = 66mm
When to Use HE
HE should be used when firing at an angled heavily armored target, or when firing at a lightly armored target. HE in almost all cases should be aimed at the targets superstructure or bow and stern sections. HE can also be used in a strategic fashion to destroy anti-aircraft guns on enemy ships to allow a friendly carrier to strike them easier.
Battleships should only fire HE at another angled battleship at range. Their AP damage is consistent enough that the HE is usually not worth using. Against destroyers, the question of HE vs. AP largely depends on the situation. A battleship that is GUARANTEED to get a shot on a destroyer should consider spending the 15-30 seconds to reload HE. Captains that need to quickly fire a shot off while a destroyer is spotted, however, should fire whatever shell type is loaded (usually AP).
Cruisers primarily fire HE. Their AP simply doesn't have the penetration required to get consistent damage on non-broadside targets, so HE is the go-to shell type. Furthermore, a cruiser's rate of fire allows them to rapidly light and re-light fires on multiple targets to stack significant fire damage. Cruisers should also fire HE against destroyers (except in rare cases, such as Tier X Russian destroyer Khabarovsk).
Destroyers, like cruisers, primarily fire HE at every target, unless they present broadside. Keep in mind that many destroyers' primary source of damage is torpedoes, and they should limit main battery usage to avoid detection.
Fires are the primary form of damage over time in World of Warships (the other being flooding), and is a vital source of damage for most cruisers and some destroyers.
High explosive shells have an attributed fire chance to them; however, it is important to note that higher tier ships have greater built-in fire resistance that reduces this fire chance. The fire chance of main battery and secondary battery shells can be increased through the use of signal flags and commander skills.