The items text file indicates that itm_ballista_launcher does 250 pierce, though I guess that may not be the real number.
Interesting. Maybe ballistae follow different rules for shield penetration than crossbow bolts? Because 250>(30+3*39). The 250p number seems plausible, because I've seen the fattest tin cans barely survive a ballista shot to the chest (would do 80ish dmg to 79 armor at range)
I'm trying to see if I can get away with a lower armor shield and still stop ballista bolts. I love charging up to a loaded ballista, harmlessly catching the shot in my shield, then hacking the operator. That always induces some rage.