Headshot has about* 20% chance of killing an enemy (except Bosses, but including their spawn) in one shot.
Headshot only triggers if the enemy hasn’t taken any damage, so the following perks make it less attractive:
- Zap and Death Zap
- Death Fire, Death Poison, Death Ice and Death Blast
- All of the Shrapnel perks, although SideWinder on it’s own is usually OK because they almost** always hit the enemy that was hit by the shot
This is obviously more useful when you need many shots to kill an enemy (because you then stand to save more shots), so is more useful at higher levels and/or when using the AutoGun or Looper; and is less useful with powerful guns like the Laser Rifle and Plasma Cannon (the latter is a really bad match because any enemy that takes blast damage is immune to Headshot)
It’s also less attractive for Hecate, Fursus, Mortis and Prospero.
* I don’t know if the number is exactly 20%, but based on observations it’s more than 15% and less than 25%, and since the game generally likes round numbers I’m going with 20%.
** The exception is when you have homing, because the SideWinders don’t home