General guideline for natural attack Claw & Bite/Slam AP values:
Size :: Claw & Bite AP :: Slam AP*
Fine** :: none :: none
Diminutive** :: 0 + 1/2 STR modifier :: 0 + 1/2 STR modifier
Tiny** :: 0 + STR modifier :: 0 + STR modifier
Small :: 1 + STR modifier :: 2 + STR modifier
Medium :: 2 + STR modifier :: 3 + STR modifier
Large :: 4 + STR modifier :: 6 + STR modifier
Huge :: 8 + STR modifier :: 12 + STR modifier
Gargantuan :: 12 + STR modifier :: 18 + STR modifier
Colossal :: 16 + STR modifier :: 24 + STR modifier
* A creature with a Slam attack will only have a base AP of 1/2 normal if it uses the appendage as a fine manipulator. Humans have only a 0 (meaning no armor piercing ability) due to the high importance of their hands. IMC the Improved Unarmed Strike feat gives a human an unarmed strike AP of 1 + STR modifier; yes, strong Khitain mystics can crush through armor if they opt to go with melee instead of finesse on open hand attacks.
** Most intelligent Fine, Diminutive, and Tiny creatures will perform their attacks as Finesse attacks, seeking to get around the armor. Even semi-intelligent creatures such as vermin will figure it out eventually, and try to get around their target's "shell."
An AP total that is less than 0 is counted as 0. Count a secondary attack as being at one size category less than the primary attack, and add only half any STR bonus. Count a fragile attack (i.e., a snake's fangs) as one size snaller. Various odd ranged attacks (like manticore spikes) are reduced by range.
This is what I plan to use for my own campaign, though it has not been tested as yet, as so far it has all been traditional hand to hand combat.
Notes from Conan Bestiary: the Grey Ape's gore attack has an AP of 8 (base 5 + 1/2 STR) which is about right. The Sabre-Tooth has a bite AP of 12 (9 + 1/2 STR, apropos of a Large creature with an effectively Huge bite). The system breaks down with no natural animals having AP scores (not even the Colossal Greater Son of Set, which has fangs the size of a dragon, and should crush armor like paper!) I'd also argue that warhorses should have AP with their hoof attacks (ever read what one of those attacks could to to even an armored knight?)
