I'm looking at the siege weapon rules from Ultimate Combat, and something's boggling me.
If I understand things correctly, when using siege weapons, you need your crew to take a number of full-round actions to aim, after which the crew leader takes a standard action to make the actual attack. This usually means there's a turn between the aiming action and the attack action.
What happens if you're aiming a cannon at a creature, and that creature decides to move during its turn? Is the aim broken, necessitating another full-round action or more to re-aim (thus possibly never allowing you to attack, if the target keeps moving)? Or have you "locked on" once you've taken your aiming actions, following the creature's position until you make your attack?
Musings: Indirect fire siege weapons target specific squares, so if a creature moves out of such a square, the attack will miss the creature regardless. Should that principle be carried over to direct fire weapons? Catapult crews could potentially anticipate where a creature would move to and then target that location, but cannon crews wouldn't be able to.
It's a turn based system that is trying to represent dynamic every moving combat.
In short, the answer is when you're aiming you're using your skill to estimate where your target is going to be. You're leading the target.
How does it work? It just does. This isn't the first place were the rules don't fit with reality, and it wont be the last. See: Magic.