Wanderlust:
I would say make it a feat with a prerequisite of Dex 13+.
Personally, I would probably increase that to Dex 15+, the same as for Ambidexterity. I would also put a BAB requirement on it. (dunno what, just something to keep it out of the hands of Lv.1 rangers)
Now then, let's take a side trek back to my original point about daggers and AP. Lets do a little bit of Q&A.
Q: Why do larger weapons manage to deal damage through armor?
Aa: Because they can deal lots of damage.
Ab: Because they get to slip through the small open spaces in it and ignore your armor.
Q: Why do smaller weapons manage to deal damage through armor?
A: They don't. Smaller weapons just suck and no one will ever use them.
IMO, one of the greatest parts of any fantasy novel is when the hero is wandering around in full plate and a rogue sneaks up behind him and plants a dagger into his kidney by poking the blade into the seam between two plates.
Also, using called shots to represent this can be a pretty bad idea in most circumstances. In my experiences, called shots end up being either brokenly powerful, just plain silly (so normally you aren't trying to kill your opponent in one hit???), or completely useless.
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If I were trying to use the system you are talking about, here is the way that I would set it up, at least initially.
Slashing and Blunt weapons I would leave alone. I wouldn't give either type much in the way of AP. (Probably no more than 3 for blunt and 0 for slashing. After all slashing attacks IRL aren't very good at penetrating even fur.)
Piercing weapons I would probably give an AP value of 2+ crit multiplier. This takes care of weapons like the war picks that were devoped for the purpose of penetrating thick armor.
Under no circumstances would I tie AP to weapon size. Doing so in either direction would nerf one end or the other.
