There it is. Outside, sitting on the walls, is a squad of marksdwarves with crossbows and quivers equipped. In the military screen, they are assigned to use any type of bolts. Just inside there is an ammo stockpile with many ammo bins. The dwarves are not going inside to pick up the ammo, and instead, they simply stay up there staring at the enemies.

For this bug: http://www.bay12games.com/dwarves/mantisbt/view.php?id=2706