I don't want to explain all the history that brought us to this point so I'll keep it simple and let you guys discuss it:
Supposedly, AP ammo deals less damage at the cost of better penetration.
Instead, normal rounds have more damage at the cost of less penetration.
We all know this is actually bullshit. Because AP is STILL actually better than normal rounds IN ALL ASPECTS. Unless the player behind the xeno is incredible stupid, cocky, or just fucks up you won't kill nothing with normal rounds, they'll just get shot as most as they can and then HEE HEE into the darkness again.
In fact, I've tested this myself playing as xeno (specifically as Lurker and Warrior) to see how much bullets I could eat before dropping. From all the rounds I've tried, only those who shot me with AP were the ones that actually made some damage to my health and most of the times, killed me. If you ask me, I feel it's kind of hypocrite to keep this as a 'well known secret' between the community and repeat what the changelog have stated (AP > Normal), because it's a lie. AP it's still better than normal round.
Should be better? Yes, in certain circunstances. Originally designed to deal with ARMORED enemies (All the T3's and some exceptions like the Defender) but instead now you have to use this 'circunstancial' ammo to deal with the entire xeno force. I'm open to debate because I've been testing this shit for weeks now.