Every type of ammo has an unique and exclusive variation, except the Pistol Ammo.
Arrows have the Bone variation, which expands the hitbox, 5.56 has the explosive ammo.
I believe Pistol Ammo could use its own unique variation, one that pierces armor.
Of course, the right value is debatable, should it be 10% armor piercing, maybe 100%? Hard to pick a number.
But since all weapons that rely on this ammo are limited to distance, I think it wouldn’t break the game to implement such an ammo type, if used for “Pistols” only.