I was checking the units stats and it appears that all archer units (mounted, foot, etc.) are "piercing" while slingers are listed as "blunt." It does not appear that archers are divided into armour piercing vs. "standard" bow, but I have not dug into this very deeply.

I find the "blunt" spec hard to reconcile with the effect some slingers have on shielded and armoured opponents at range. Of course, I'm thinking specifically of the Balearics, and they have really high stats, so perhaps this is an unfair representation of slingers in general.