Wait, the bow will have AP…?
I’d say AP should pretty much always be a property of ammunition, not the weapon firing it.
Now, having bonus AP as % of base damage dealt sounds much more promising:
Let’s say AP bolts would have 50% extra AP bonus: so whenever you shoot one, they get effective armor penetration of 50% of the damage it was about to deal.
So if you get a good shot from a strong crossbow that deals 30 base damage you get 15 points of AP to negate armor before said armor reduces damage.
But if you fire the same bolt and only deal 6 base damage, you only get 3 points of AP.
Etc, etc.