So I have noticed that many guns have an option for an armor piercing modification which is usually coupled with full auto. My question is what does this armor piercing actually do? When equipping armor piercing with full auto you get a pretty significant damage reduction. Does this mod only work more effectively on certain enemies that have heavy armor and is less effective on unarmored enemy types? There are also some melee weapons that have this feature as well but they don't really seem to cause extra damage or in some cases they deal less damage than the other mods. In the end I see absolutely no reason to equip these armor piercing mods as they seem totally useless and full auto as well is just a big waste of ammo in most cases.

i am guessing they do more damage to armor. ignoring some DR or is DT. there is a perk that ignores some of that to.

I find it frustrating that in order to get the armor piercing bonus you need to go full auto. Especially if you focus on a single fire perk tree like rifles or pistols. My guess is armor piercing mods are only worth it if you use the commando (full auto) perks.

It would be interesting if someone could do the math and explain whether the armor piercing bonus was good enough to counter the damage reduction.

The reason that armor piercing is only on full auto weapons is because they're the ones that need it. Full-auto weapons are hurt more by armor because more of their damage is negated, so an armor-piercing mod makes these weapons more effective than a straight damage boost would.