I don't think there is a listed weight. But since a Light Ballista is "essentially a Huge heavy crossbow" (from the description of the Ballista), you could extrapolate the weight from a Heavy Crossbow.
The 'Armor for Unusual Creatures' Table tells us that Huge armor should weigh five times its normal weight. So if that is the same for weapons then a Light Ballista should have a weight of 40 lbs, the same as a huge Heavy Crossbow.
Well, Ultimate Combat says that the ammunition for a light ballista weighs 10 lbs per bolt. A normal bolt for a crossbow weighs 1/10th of a pound.
So extrapolating based on this, we could say that a light ballista is 100 times heavier than a crossbow, so probably 400 lbs for a light ballista if it's an upscaled light crossbow or 800 lbs if it's an upscaled heavy crossbow.
You generally don't see weights because they aren't portable.
So are you a cloud giant or taller? Then you can move a light ballista.
If you're not as tall as a cloud giant, you either have it bolted to a wagon or a castle wall, in which case the weight isn't an issue.
You generally don't see weights because they aren't portable.
So are you a cloud giant or taller? Then you can move a light ballista.
If you're not as tall as a cloud giant, you either have it bolted to a wagon or a castle wall, in which case the weight isn't an issue.
I could see a group containing a telekineticist wanting to know how much one weighs to see if it can be handled by Telekinetic Haul as an alternative to the wagon.
As a general rule, two-hand weapons are one size category smaller than the appropriately-sized wielder, one-hand weapons are two size categories smaller, and lights are three size categories smaller. A light ballista is size Large. At a minimum, the creature carrying it needs to be Huge. As a GM, I would not allow it to be mounted on an animal companion smaller than size Gargantuan without a significant penalty on the attack roll.
The best-case scenario for most PCs would be to have it on wheels with the animal companion pulling it.
IF you co by the core rulebooks suggestion that a ballista is essentially a heavy crossbow sized for a Huge creature*, and use correct scaling (1 size large is double in all dimensions, so 8x weight not 2x), that puts a Huge crossbow at 64x the weight of a medium one. Medium heavy crossbows are 8 pounds, so the Huge crossbow/ballista would be 512. (Similarly, if you apply the same scaling to the bolt, you get one that weights 6.4 pounds).
If you stick with a 10 pound ballista bolt (100x the weight of a heavy crossbow bolt), that would suggest an 800 pound ballista (100x heavy crossbow weight).
*Both deal 3d8 damage, 120 foot range increment.
I know it is a different game system, but supplements for GURPS are generally somewhat well researched (unlike D&D/Pathfinder, which just slaps a number on paper that sounds good), at least the low tech ones. There, a ballista/scorpion that fires a roughly 6 pound shot (roughly what a "proper" scaling of a huge heavy crossbow gives) weighs about 600-700 pounds.
As a general rule, two-hand weapons are one size category smaller than the appropriately-sized wielder, one-hand weapons are two size categories smaller, and lights are three size categories smaller. A light ballista is size Large. At a minimum, the creature carrying it needs to be Huge. As a GM, I would not allow it to be mounted on an animal companion smaller than size Gargantuan without a significant penalty on the attack roll.
The best-case scenario for most PCs would be to have it on wheels with the animal companion pulling it.
That is wrong.
A weapon's size category isn't the same as its size as an object. Instead, a weapon's size category is keyed to the size of the intended wielder. In general, a light weapon is an object two size categories smaller than the wielder, a one-handed weapon is an object one size category smaller than the wielder, and a two-handed weapon is an object of the same size category as the wielder.
Crossbows can be fired 1 handed (though at a penalty). That would suggest that a crossbow is one size category smaller than that of the intended wielder, not the same size. So a crossbow intended for a Huge creature is itself a Large object. Which is what Ultimate Combat says a light ballista is.
This is what I get for going off memory instead of looking it up.
We have mounted ballista-like weapons on the back of war elephants before. So there is a real world precedent for the same concept.