When you score a Critical Hit against a creature using a Ranged weapon, you can choose for it to be a Headshot. If the creature has less than 100 Hit Points, it dies. Otherwise, it takes an extra 10d10 damage of the weapon's type.
Once you use this feature, you can't use it again until you finish a Short or Long Rest. You can also restore your use of it by expending three ValdaGunslinger (no action required).