Highest damage via a ranged attack? (non-spell)

[ksbsnowowl]

It uses a few unupdated 3.0 sources, but check out the archer build thread linked in my sig.  It got something in the realm of 300 damage as part of a single attack.  It was general, but Dragonslayer could aid in making something similar against dragons with less 3.0 material.

Ah, here is the link:
http://brilliantgameologists.com/boards/index.php?topic=194.0

There are a couple incarnations in that thread, so scroll down through it.

[Ithamar]

If it doesn't have to be a bow, you might make good use of Bloodstorm Blade, or the Lightning Throw manuever.