Hm I did some testing with Slinger vs Archers, I will try to found the sheet with the results somewhere...

I don't know anymore what exactly contained the projectile_desc. but if my memory doesn't trick me it seems that there was a explicit armor_piercing stat.

Anyway it seemed that good Slingers are better suited for masses of lighter armored targets while good Archers are better to decimate heavily armored enemies. I tried BS and Chosen Archers against LevyPikemen and SSP with upgraded armor...

Shields seemed also more important than in MTW, as equally well armored Agema Phalangites (Royal Pikemen) had far less casualities per volley as I can recall

Perhaps the sheet with the results will turn up...

Cheers
OA