Sounds like something ideal for DM hand-wavium, personally. What're you trying to do with one?
If you're trying to breach a wall, DM hand-wavium for the dramatic moment.
If you're trying to take down a dragon, DM hand-wavium for the DC to aim it, standard action to fire it, standard action to load it, and DM hand-wavium for how many bolts/rocks/cows have to hit to kill it.
If you're trying to lug it through a dungeon to use in standard fights, rocks fall, everyone dies. They're not for standard fights.
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