Aight I'll simplify: When you hit someone with a MC attack (sting, VS) your damage to them is calculated as (your damage - their AC).
Now, if you your damage is less than their AC, you will simply not damage their health, only their armor.
If however, you're carrying a B12, or punching, these abilities have a % armor penetration built in, which basically means that even if you do less damage than the enemies' AC, you will still hit through their armor, these values are approx 30% for UA attacks and 5% for a B12 as Nao stated earlier.