Zander said:
Are there any rules for hitting particular parts of the body? If so, where (book and page #)?
What's the penalty on your attack roll to hit someone on the head?
You could:
1. Work it like the old Carrion Crawler. Give one AC for attacking the face and another AC for general attacks.
2. Work it like "Striking an Object". Or, specifically, like striking a worn helm that isn't actually there.
Attacking a held, carried, or worn object provokes an attack of opportunity. Objects that are held, carried, or worn by a combatant, are harder to hit. The object uses the combatant's Dexterity modifier (not its own –5) and any magic deflection bonus to AC the combatant may have. The attacker doesn't get any special bonus for attacking the object. If the target object is in the opponent's hand, it gets a +5 AC bonus because the opponent can move it quickly out of harm's way.
Table: Size and AC of Objects
Size AC Modifier
----- -----------
Colossal -8
Gigantic -4
Huge -2
Large -1
Medium-size +0
Small +1
Tiny +2
Diminutive +4
Fine +8
Treat a head like a Tiny object. The final AC will be 10+Dex bonus+armor bonus if wearing real helm+2 for Tiny "object". And definitely make this attack provoke an AoO.
3. Use a random hit location table with each strike (tip: use a different table for piercing attacks).
4. Make it so a natural 20 = head shot.
I'd refrain from writing a "random head shot chance" into your rules. If a head shot has a special effect upon something for plot reasons, then make the PCs deliberately target the head (I recommend the "attack an object" rule). Don't leave it up to chance.
In fact, I'd just go with this:
5. Make the players describe each attack. If the player describes that he's attacking the head, and rolls a hit, then guess what: he just hit the head. If player is not specific, assume he or she picked a target other than the head.
-z, who prefers hearing players say "I swing at his legs! Then I reverse the motion, coming up under his arm!" instead of "I attack twice. I hit. I miss." Or, worse, "16. 12. Does the 16 hit?"