Is there a rule for this?

[Yogibear41]

Suppose instead of actually holding and shooting a large or bigger heavy crossbow as a medium sized character, you set up a stand or tripod if you will to position the crossbow on and just turn it while that supports it to fire. Would this in anyway make up for it being to larger for you to normally use?

[Gavinfoxx]

Suppose instead of actually holding and shooting a large or bigger heavy crossbow as a medium sized character, you set up a stand or tripod if you will to position the crossbow on and just turn it while that supports it to fire. Would this in anyway make up for it being to larger for you to normally use?

Nope. Ballistas take penalties to fire due to their size. You would need Oversized Weapon (Ex) to overcome this, even with stands or supports. I believe Titans have Oversized Weapon, but only for their warhammers.

I added it while homebrewing this creature:

http://www.giantitp.com/forums/showthread.php?t=271165

[unseenmage]

I don't remember whether it's in the old Stronghold Builder's Guide or the Arms and Equipment Guide but one of them has ballistae and the rules for operating them.

And as bigger weapons of the same stripe are just bigger weapons in D&D that might be as close as you'll get to a rule for what you're asking. But I could be wrong.

[lsfreak]

You might be able to cross over into siege weapons territory and use Profession (siege engineer) instead of an attack roll, as found in Heroes of Battle. It's been a while since I looked at the rules, but from what I vaguely remember, they don't have an equivalent of a scorpio (http://en.wikipedia.org/wiki/Scorpio_%28weapon%29), which is more like what you're looking for, and if I'm not getting my rules confused it has you targeting like a splash weapon, and thus allows reflex saves, as well. But it might be a place to start.

[aeauseth]

You might want to review the ballista rules:

A ballista is essentially a Huge heavy crossbow fixed in place. Its size makes it hard for most creatures to aim it. Thus, a Medium creature takes a -4 penalty on attack rolls when using a ballista, and a Small creature takes a -6 penalty. It takes a creature smaller than Large two full-round actions to reload the ballista after firing.

A ballista takes up a space 5 feet across.