|This article is about the current version of DF.|
Ballista arrows are essentially giant arrows, created at a siege workshop and used as ammunition by a ballista. There are two types: simple wooden ballista arrows, which are made by unit of wood at the workshop, and much stronger metal ones, which first requires the creation of a ballista arrowhead at a metalsmith's forge and then final assembly with wood at the siege workshop.
For such a big, cumbersome, inaccurate weapon, wooden ballista arrows are surprisingly ineffective, dealing only comparatively minor amounts of blunt damage. Metal ballista arrows, on the other leg, are extremely lethal, and can kill two fully-grown elephants standing in a line if the arrow hits squarely; any goblins you hit will be smeared all over the space on which they stood.
Creating arrowheads is tedious and resource-consuming, and both catapults and ballistas train the siege operator skill, at the same rate, when used. Since catapults use stones you can literally pick up off of the ground, they're a much better practice platform for your operator, whereas a ballista armed with metal arrows is a much stronger actual weapons platform. As such there's never much reason to produce wooden ballista arrows.