Called Shot

Called shot is a very specific ranged attack, such as Throwing knives, long or short bows, throwing axes, and darts. It can not be used for magic attacks. It is meant for a specific location on a creature, or other adventurers. Shot must be declared prior to rolling for attack, and DM must be aware of this type of attack. 
When making a Called Shot, the attack is made at disadvantage. An attack roll of Nat 18, or 19, results in a double damage hit. But the player can also add the modifier for Strength to the attack modifier if it is a Dexterity based modifier, and Dexterity to the Strength based modifier. As well if the attack used your Charisma modifier in place of Strength or Dexterity, such as with a Warlock, the Dexterity or Strength modifier would be added to that. If a Nat 20 is rolled the damage is tripled plus, normal modifiers as described before. If the attack would other wise hit the damage is treated as normal, with normal modifiers.