What is AP in game terms is going to be on how the attack is bought(character points or money)
If I pay character points it is AP if I pay for it and the GM approves
If I pay money I should have to pay x10-x20 vs normal arrows and the GM approves
to a lesser degree if paying money for an AP weapon say x5-x10 vs a normal weapon, and the GM approves
In any case the classical case to deal with heavy armored foes is volume of fire
Example a force of 50 heavily armored knights(armor act-14)attacked by 200 archers should yield say 25 hits(area fire in reality)
14- activate is around 90% effective defense
so of the 25 hits 2.5 will penetrate for an average of 5.5 body done(1.5d6 weapon) to the poor knight hit(probably taking out something and making the knight combat ineffective
it now becomes a race for the knights to cover the distance to the archers and destroy or bypass the archer's defenses(trench with emplaced spears, high ground, defensive toops, etc...)
as for a rapier being AP, I'd say no as it is a weapon meant to slip into the joints and do damage( though a rapier is a street/ in the city weapon and is not meant for the battlefield)
Now I would either buy AP as a naked advantage or Penalty Skill levels to lower the activation roll to show the added skill of the wielder