Originally Posted by Dano
General rule is
If you are a BB and firing at anything lower than you use HE
BB to BB AP or HE depending on distance and angle.
Striaght on shot HE or extreme range.
If you are a CA and firing at anything lower use HE
CA to CA AP
DD always use HE
I'm going to disagree with part of this.
BB's (except the Arkansas Beta) should be firing AP at everything except DD's. AP will over-penetrate the destroyers and do minimal to no damage. I exclude the Arkansas Beta from that rule because she does as much damage with HE as she does with AP, sometimes more.
Because of the reload times for BB's it isn't very profitable to switch ammo types when fighting other BB's
For Cruisers (CA) You have to be more picky about your ammo choices vs opponents. AP works well against most CA's but not against the St. Louis and Bogatyr (Tier3) as they are both protected cruisers and have surprising AP resistance. Unless you are in a heavy gunned Tier5 or higher cruiser I would suggest using HE against those two targets. And the only tier v cruiser that's gunned heavily enough to be AP effective against them is the Furutaka which they won't encounter with any regularity unless MM is drunk.
The nice thing about the main batteries on CA's is their reload speeds are fast enough that you can swap ammo types efficiently to alternate between HE (set them on fire) to AP (hit them from criticals and citadels) and back to HE (set them on fire again and use their repairs up)
Anything shooting at DD's (inclusing other DD's) should be using HE pretty much exclusively. The higher tier DD's (8,9,10) have more utility with AP but the low tier DD's will almost never be effective with it.