Yes armor piercing adds damage, but very little like 2 damage points in PvE. It also does go through enemies- err it goes through one enemy at a time anyway. Before the armor piercing round nerf it was really fun against mobs of PvE enemies like Thralls. Not to mention in PvP I tested it on people gathering around heavy ammo, it went through 4 people that were around it O.o. Obviously not all 6 were there, but you get the point of the armor rounds and how they worked before the stupid nerf. Now armor piercing rounds got nerfed to the point where it only goes through ONE count it ONE enemy and stops at the one behind him/her... your better off using stability. The added damage is just piss poor, and now the main part of armor piercing is just garbage now.

It increases damage by like 2 or something, but it means you can shoot two people with one shot (if they're lined up correctly)