Concerning measurements, I copied this from my notes:
So, the length of the dart or Kestros is, according to:
Polybius: 2 palms (~6") + 1 span (~9") = ~15"
Livy: 2 palms (~6") + ½ cubit (~9") = ~15"
Hawkins: ½ elle = 15"
Hondero, I added the point dimensions under the photo. The longest one is 15 cm as Polybius says. Then again, maybe his 'palm' was smaller?
Johnny, Sorry, I don't want to take on another project!
Gun, Livy said the fins were made from firwood.
Crossbow bolts of the era were also about 15" in length. It is likely, I think, that the Kestros is derived from the bolts used for the gastraphetes (belly bow). Slingers would have had access to bowmen.