Actually, just make it a tag. IE
Shotgun and fire has [shred] tag, so it does 80% less dmg to Hard bodyparts (like most automatons or armor).
Most assault rifles have [pierce] tag, so they do 80% less dmg to bug butts and automatons' guts.
Explosives have [explosion] tag, which doesnt suffer from this dmg reduction.

This is also good because now you can make the assault rifles different from each other, by simply changing a tag (cuz some are hollow point, other armor piercing). Very good, since currently all assault rifles are the exact same gun.