It works well on undead and fiends. Fiends include devilites, which dodge, and difficult targets like greavers and wings, so a gun with a small hitbox isn't amazing against them.

Unfortunately, because of the split damage, when it does get a bonus, it's half of the bonus a fully typed weapon would get. The split damage also means that it's effectively penalized against all other monster types - the 3 weapons with a split type pairing that isn't half normal (AP, Sentenza, Radiant Sun Shards) never deal 100% of their base damage, which is... somewhat odd, really. They're also an awkward fit in your arsenal, because they require you to have a weapon of the only type they're not (or normal) to cover all possible monster resistances, where any pure weapon goes with any other pure weapon of a different type.

So AP really needs your backup weapon to be pure shadow or normal to work anywhere outside of FSC. FSC is the main reason people get AP, mainly because nothing in that entire dungeon resists Elemental(except the one spawn of wolvers and, I think, Vanaduke's mask?) and only the gun puppies resist Piercing, so the usual drawbacks of using it don't apply there - nearly everything is weak to it. In any other strata that isn't undead or fiend-themed, however, you can't rely on it, and even then arenas can screw it up.

In pure damage terms, AP generally works out weaker than other guns, especially when you take into account the alchemer's splitting bullets or pulsar's expanded shots. It's only real advantage is mobility, which is why it sometimes gets chosen over other piercing guns for Vanaduke.