What is ballista arrow impact zone?

I made second entry to my fort as 1x100 tile long corridor, at the end of it I have ballista installed. It uses bronze ballista arrows. With first arrow, 10-15 goblins killed, but second arrow misses most of remaining after first arrow goblins.

1. What are rules for flying ballista arrows?
2. Will it hit lying target? (for example if first arrow demolished legs, or creature collapsed from injuries)
3. If several creatures occupie one tile, will it hit all of them or only standing one?

1. Unknown at this time, experimentation is key!
2. Yes, keep firing!
3. Yes! Die goblins die!

Spoilers ahoy!

Watch out when goblins are 'half down' or dead ones lying around while still firing as someone might try and loot them - ballista's don't care who they hit!