- After killing an enemy with a headshot, your next weapon hit within 30s deals 125%(150%) of that killing blow’s damage in addition to it.
- This damage is capped to 1500% of your weapon damage.
The first part is easy to understand, you deal 100 damage on a headshot kill, the next shot you get 125 bonus damage.
The second part is a bit weird, let's say your gun has a base damage of 100, and you have 100% headshot damage, so you hit for 200 on the first headshot. You would expect to get 200*16 = 3200 damage as your damage cap, but it is actually way lower. What you will get is actually 1700, because the bonus damage is capped at 1500% of your weapon damage, and cannot be scaled with most things.
Here are the things I tested:
Things that can scale the bonus damage : Weapon damage, Amplified damage (tested with Versatile)
Things that don't scale : Headshot damage, Crit damage, Damage to armor, Out of cover damage.
So basically despite it stating you need to head shot with the skill, building headshot damage isn't really efficient. Get the most base damage you can, and maybe you can fit in some amplified damage like Versatile, Aces or Pulse Overcharge.
Lastly the bonus damage seems to apply to every pellet on a shotgun, so you can probably one hit everything in the game that doesn't have invunerability phases.