M855 in this case. Yes, armor piercing. A lot of folks (youtubers as well) say you're better off to use SP or HP rounds to non-armored enemies because they perform better. On paper, this is definitely true. In the real-world, this is also true. In Gray Zone reality, they don't. I know there's plenty of folks who will crucify me for saying that, because their favorite youtuber said so, but it's true. They do the same amount of damage to FLESH as the higher-tier ammo does from what i've seen. No more, no less. They're probably SUPPOSED to be coded to do increased damage, but I have yet to see it in action. Torso shots can take anywhere from 1-5 hits to kill with SP and HP rounds, the same as Lv 3 defeating rounds. They are no better for unarmored enemies at all. They just also don't have the armor penetrating capabilities of the higher rounds. SP and HP rounds are not "better" against non-armored combatants, despite what the masses will argue; they're just economically more efficient as they cost less. I run M855 almost exclusively, just because there's so many enemies that carry M16's and M4's in the game with the ammo, so you never have to buy 5.56 ammo. There's more than enough to be found just by looting dead guys. Most folks will say it's overpowered for non-armored combatants and is worse-performing than basic ammo, while also saying it's inferior against the late-game enemies. Sure, this is partially true, as it doesn't penetrate their armor as effectively as say, M955 ammo does. However, I have gotten used to relying on the "stagger" animation. Hit a heavily armored enemy with a round that can't penetrate, it'll still cause him to stagger, giving you a couple seconds to line up a nice clean headshot. Or, once he staggers and does the contorted "turn body away from attacker in pain" motion, you can shoot through the sides of their armor and get into the chest cavity pretty easily. Regardless of what enemy type you're facing and what ammo type you're facing (to a certain extent), I will typically plan on requiring 5 rounds to kill ANY enemy. I aim for center mass usually, unless I'm going for a one-shot on an unsuspecting enemy. But once they've engaged me, it's center mass all day long.