World of Warships ammo comes in two types - Armour Piercing (AP) and High Explosive (HE).
Generally AP has higher potential damage per shell if it penetrates the target's armour and especially if it penetrates the enemy ships citadel but can richochet if it hits at an extreme angle or simply bounce off heavily armoured ships or do minimal "spalling" damage. It's also possible to overpenetrate very lightly armoured ships such as destroyers doing minimal damage as the shell punches clean through the ship and out the other side.
HE does less potential damage than AP but is more consistent and has a better chance of knocking out modules and setting the enemy ship on fire for damage over time effects.
New players should stick to HE if they are not sure what ammo is appropriate as it's always at least somewhat effective but AP is more situational. More experienced players can consider the following information.
The general rule is load HE whenever you are not sure what ships you are going to encounter. Thereafter you should consider the following rules of thumb.
- ALL ships should use HE against aircraft carriers if they get into firing range - AP is likely to overpenetrate most carriers and HE can set the flight deck on fire - while burning Carriers cannot launch or recover aircraft. This is particularly true with USN carriers that lack armor to deflect even the smallest HE shells.
- The exception is with certain IJN carriers such as Hiryu and Shokaku. These ships should be hit with HE to set them on fire first, and then to AP in order to deal fair damage. Most AP can penetrate and deal good damage if aimed correctly.
- IJN Destroyers such as Mutsuki or Kagero should normally consider their guns as secondary weapons anyway and concentrate on torpedo strikes. Firing your guns makes you an easy target for any cruiser in the area, and considering the IJN Destroyers have poor guns compared to the USN or VMF it is wise to remain silent. If you do fire your guns, use HE. Your AP shells are unlikely to do much to enemy cruisers, much less battleships and if you're firing at enemy destroyers your priority is going to be knocking out their turrets and torpedo launchers so they can't fire back. Of course there are certain times and ships that may be exceptions, such as shooting a Kuma below 5 km.
- USN and VMF Destroyers like Farragut or Kiev often have poor torpedo ranges, and therefore guns should be your main weapon. The general rule of thumb is still HE on practically every ship, except cruisers below 5 km and you have prior knowledge of the weak areas in the armor. RN destroyers have much better shell ballistics than USN or IJN, and AP may be a good choice against all cruisers below a certain range.
- Light Cruisers such as Cleveland and Omaha should fire HE at enemy destroyers - AP is likely to overpenetrate. At enemy battleships you should use HE and try and set them on fire. Certain battleships have some armor on the superstructure, and AP shells can do fair damage if they are already on fire. Against enemy cruisers it depends on their relative armour and your gunnery skills, AP is the best choice for most cruisers but HE them if you are not sure.
- Heavy Cruisers such as Pensacola or Aoba should stick to AP against all other cruisers. HE is still the most effective way for battleships and destroyers.
- Battleships should use HE on destroyers and AP on cruisers and battleships. However, if you are outmatched in a battleship on battleship duel such as you as a Kongo facing a New Mexico, use HE since your AP shells will deflect off of the well-protected New Mexico.
Note: Usually around Tiers 4 and 5 is when ships start getting better "Stats" Thus it is usually when you start to see Heavy cruisers and more heavily armored battleships. Past Tier 5 is when more players use AP more often due to the game being more challenging and ships having better overall stats and upgrades.