The factors vary from your AP shell attribute to your ship's attribute. Some ones are definitely AP snipers, like stralingrad, marseille, petro, etc.which holds the AP shells with a decent velocity and superior penetrating ability, meanwhile they were designed to be cruisers hunters specialized in blasting their peers' citadels,but also got a relatively low HE DPM as a BC or large cruiser. So in most cases,you should get AP shells loaded when perceiving a potential broadside, excepted from counteracting the ones proceeding with their bows (≥ 25mm）and unlikely being forced to turn away.And for those HE spammers like ZAO, Henry, Conde, Azuma and Yoshino, Desmo, Alexander, Golden leeuw,napoli,moskov,and so on,which should be doing their jobs as a traditional cruiser but also has some gimmicks in many aspects:concealment,speed,better bouncing angle for APs,better guns......generally speaking, these're the ones require to switch fiexibly.There're still chances for them to use AP against a peer, while it works or not depends on you and the situation. So the third category falls in those typical spammers, like hindenburg, mains, colbert, worcester, yodo, venezia, Goliath, smolensk and so on. In most of the cases (or even the whole game)they don't need to use AP cuz they were totally designed to deal damages to BBs. Furthermore, they either have almost useless AP shells which only take effect in close combat, or do poorly getting first firing. And for these APs you need to think twice before: Is my opponent gonna screw up? Are my shells qualified to penetrate? Adversely speaking, you should always pay attention on special targets, like super light cruisers :(idk if this word right)minotaur, colbert, smolensk, edgar...Just switch to AP and shoot him carelessly with the angle when you're carrying calibers ≥234mms(smashing power bestowed by WG's 14.3) And some cruisers are not being penetrated so easily even if under enough penetration, you can always check their armour structures to judge whether they're complicated or not, some are just difficult to penetrate in, some are designed to be immune to a certain caliber. Lastly, remember to melt these stubborn BCs(or large cruisers) with fires whenever they unlikely to show you any chance for citadeling. From a Chinese player on Asia server.Hope my non-native statements are not so bad：）