There’s a good amount of debate among avid D&D players when it comes to Called Shots. For me, they offer more realism and creativity during combat, which in my opinion, only makes combat better. You can read my more in-depth discussion here.
The purpose of a Called Shot is to allow a PC or NPC to target a specific part of someone’s body for some special effect. I started out first by thinking about appropriate AC bonuses for Called Shots. Obviously we don’t want a player saying, “I’m going to hit the unarmored portion of that soldiers face” so that they can negate his AC bonus. That makes hitting a smaller target easier, so we need to apply bonuses according to the size of the target. This can be done easiest, simply by using the size category modifiers D&D has given us:
Small: +1 AC bonus
Tiny: +2 AC
Diminutive: +4 AC
Fine: +8 AC
I would also recommend adding modifiers for +3, +5, +6, and +7. A player might argue that the AC bonus to the defender shouldn’t include their Armor bonus, as the PC is specifically aiming for an unarmored spot. My response to this would be that armor should be thought of as an AVERAGE protection over the entire body AT ALL TIMES. Unless a target is completely stationary, I see no good reason to ignore their Armor bonus.
Now that we have the AC modifier, what fits into what size category? That’s a matter of judgment, but when speaking about your average humanoid I’d say aiming for something like the eye would fall under Fine, while a head shot might be Diminutive or smaller if they have a helmet. So what happens when a player is successful? Here’s a chart I’ve worked out to determine some specific effects:
Head: +6 Damage, Fortitude Save DC 18 or Dazed for 1 round if 10% HP is lost
Neck/Throat: +8 Damage, an attack to the throat renders the target speechless for 1d4 rounds if 20% HP is lost, Fortitude Save DC 19 negates
Eye: +10 Damage and Blindness (Blindness penalties are halved for 1 eye). Blindness is permanent if target loses 50% of their HP from the attack, otherwise it lasts 1 round (No Save)
Arm/Hand: -2 penalty to attack rolls with injured arm if 20% HP is lost, Fortitude Save DC 16 or items are dropped
Leg/Foot: Lose half DEX bonus to AC and -10% Move if 20% HP is lost, Fortitude Save DC 15 or target falls
Feel free add any number of other effects to this list and change the specifics to suit your own campaign. The most important rule of being a Dungeon Master is to be creative and bend the rules often. You might also consider calling for a Full-Round Action to make a Called Shot if you find it’s making combat too easy for the players.
I also like to offer more severe results when a target is reduced to 0 HP. Generally, this means something is severed, and in the case of a headshot, it’s often instant death. One important thing to keep in mind with all of this is that if your players are determined to use Called Shots, you should not feel bad about using them against the PCs as well in order to maintain game balance.
So have fun, and go shoot someone’s eye out.