The AC-130 is a GUN ship. They don't use no steenking missiles.
They could write messages on the 105 or 120 mm shells though. Or on the 40 mm . If they write really small, they could write on the 20 or 25 mm Gatling rounds. U model, aka Spooky, has the 120 mm and the 25 mm, while the H model, aka Sceptre, has the 105 and 20 mm guns.
New Somalia thread up, but I don't think it's discussing the air attacks:
http://www.freerepublic.com/focus/f-news/1764499/posts