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DMG page 255-256 describes several siege weapons which are actually objects. The siege weapons require actions to load, aim, and fire them. For the ballista, for example, it says:

It takes one action to load the weapon, one action to aim it, and one action to fire it.

I have always imagined you have to stand within 5 feet of the object and then take the "load", "aim", or "fire" action.

Part of the description for the mage hand cantrip says:

You can use your action to control the hand. You can use the hand to manipulate an object, open an unlocked door or container, stow or retrieve an item from an open container, or pour the contents out of a vial.

So technically the mage hand can manipulate an object - but can it "load", "aim" or "fire" a ballista?

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  • \$\begingroup\$ Does my answer solve your problem well enough for a green check? \$\endgroup\$ – Thomas Markov Mar 8 at 23:21
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Mage hand is intended to be able to perform simple manual tasks.

The spell description of mage hand says (emphasis mine):

You can use the hand to manipulate an object, open an unlocked door or container, stow or retrieve an item from an open container, or pour the contents out of a vial.

Here, the PHB outlines the rules for interacting with objects in combat. Some notable examples of things you can do with your object interaction are:

  • draw or sheathe a sword
  • open or close a door
  • withdraw a potion from your backpack
  • pick up a dropped axe
  • take a bauble from a table
  • throw a lever or a switch
  • stuff some food into your mouth

It is important note the simplicity of these actions - these actions typically would not require any sort of ability check to perform them without the use of mage hand. Actions requiring more dexterity are not intended to be doable with mage hand - this is evident in the Arcane Trickster's ability Mage Hand Legerdemain:

Starting at 3rd level, when you cast mage hand, you can make the spectral hand invisible, and you can perform the following additional tasks with it:

  • You can stow one object the hand is holding in a container worn or carried by another creature.
  • You can retrieve an object in a container worn or carried by another creature.
  • You can use thieves’ tools to pick locks and disarm traps at range.

By using the phrase "additional tasks", this ability description implies that the list given in the mage hand spell description is intended to be a more restrictive list, and this seems quite natural. These additional tasks, if performed without mage hand, would also require the associated ability check to be made, as described in Mage Hand Legerdemain:

You can perform one of these tasks without being noticed by a creature if you succeed on a Dexterity (Sleight of Hand) check contested by the creature’s Wisdom (Perception) check.

These particular tasks notably require more dexterity than the more vanilla object manipulations, as evidenced by the required dexterity check to perform them unnoticed. This points to an intentional lack of dexterity of the vanilla mage hand that Mage Hand Legerdemain makes up for.

A broad reading of mage hand would seem to imply anyone could do these things anyway, as this apparent lack of dexterity is not sufficiently communicated in the spell description of mage hand, yet the Arcane Trickster has an entire class feature dedicated to them.

But what about operating siege equipment?

It's up to the DM.

Keeping in mind the intended limits of mage hand, it's going to be up to the DM to determine if we can fully operate the ballista with mage hand. Load and Fire seem to be, at least to this DM, simple enough manipulations that a mage hand could perform, assuming that the ammunition and reset force is less than 10 pounds and the firing mechanism is simple like a trigger or button - more complex machinery will likely fall afoul of the arguments made in the previous section. A possible rule of thumb (rule of mage thumb?) I would suggest would be:

Could the tasks of loading and firing be reasonably performed with one hand without requiring any sort of dexterity check?

If it's simple enough to be performed with one real hand with no issue, it's probably simple enough to be performed with one mage hand with no issue.

But, I would rule that aiming is beyond the limitations of mage hand for a few reasons. First, siege equipment is quite large, and often lacks fine manipulations for aiming, and where there are fine aiming instruments, they may require a dexterity that mage hand is not intended to offer. I would rule that it takes more strength than a mage hand can muster to aim siege equipment. Further to this, there is also the matter of what aiming entails. It requires a field vision that mage hand doesn't afford. If you are in a position with field of view to meaningfully be able aim the siege equipment, you're probably not using mage hand anyway, you're moving it yourself.

Conclusion: You might be able to load and fire, but probably can't aim with mage hand.

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    \$\begingroup\$ I think that loading a ballista means also to pull the "bowstring" (by some leverage, maybe?), since a ballista is like a huge crossbow: the limit of the weight applies also on this in addition to the weight of the ammo. \$\endgroup\$ – Eddymage Aug 25 '20 at 16:05
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    \$\begingroup\$ @Eddymage Cranking a winch, yeah. The draw weight on a Ballista is way too high to expect people to pull it back by hand. \$\endgroup\$ – guildsbounty Aug 25 '20 at 16:37
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    \$\begingroup\$ @SammetDelücks I think Please Stop Being Evil's point is that thieve's tools are an object and naturally would seem to be included in mage hand's "manipulate an object". It is not at all clear from the description of mage hand that I shouldn't be able to "retrieve an object in a container worn or carried by another creature", but the description of the Arcane Trickster's ability make it seem that mage hand is naturally unable to perform such a feat of legerdemain. \$\endgroup\$ – Thomas Markov Aug 25 '20 at 19:04
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    \$\begingroup\$ @Pleasestopbeingevil Manipulating an object should not include anything that requires an ability check, such as using tools to perform skilled tasks. Fast hands has nothing to do with this; it specifically lets you open locks or disarm traps as a separate item from Use An Object. Similarly, the Trickster's ability specifically lets you open locks or disarm traps using a spell that normally can't do stuff like that. The exception proves the existence of a rule; if they felt the need to create an ability to let you pick locks with mage hand, clearly mage hand can't normally pick locks. \$\endgroup\$ – Darth Pseudonym Aug 26 '20 at 18:24
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    \$\begingroup\$ @RevenantBacon I'm not sure, I'm not aware of an explicit statement of such a restriction. \$\endgroup\$ – Thomas Markov Aug 27 '20 at 13:23
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This is a more common sense approach, since nothing I write here is explicitly mentioned in the rules. But if explicit rules are absent, we should still go by what is physically possible.

If you look at actual ballistae you will see that they are not single-handed weapons. They aren't even single person weapons. To properly operate them in useful speed, you need four hands of two coordinated people. Four uncoordinated hands probably won't do, less than four hands are definitely slower than normal, with some ballistae even impossible.

Taking the projectile alone, it seems that their weight was heavier than what a single mage hand can carry. It was measured in talents, with one talent being a normal weight in the Roman era. That's around 26 kg per piece of ammunition, way more than the Mage hand is allowed to move. So the Mage hand cannot even put a new bolt or shot onto the ballista. If the ammunition is too heavy to lift, that beats any rule saying the Mage hand can take a loading action. A loading action describes how long it takes to do something otherwise possible, it's not a magic trick making otherwise impossible things possible.

To sum it up: By and large a Mage hand can do things a person can do easily with one hand. That is what the rules try to simulate. Operating a ballista is not one of those things. It needs multiple strong human hands. So a Mage hand cannot do it.

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    \$\begingroup\$ Typical weight for a Roman ballista shot, according to the wiki page, is 26 kg, so much more than the about 5 kg allowed by mage hand. Adding some of these numbers might improve the answer. \$\endgroup\$ – Tommi Aug 26 '20 at 10:57
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The most troublesome aspect would be the weight limit on mage hand.

The hand can't attack, activate magic items, or carry more than 10 pounds.

The DMG doesn't list weights, but any siege weapon ammo can be expected to weigh more than 10 pounds, so loading is out. Aiming will probably require exertion of more than 10 pounds of force to direct it.

Firing might depend on the siege weapon. Does firing it require simply pulling a lever? Then, sure, I suppose mage hand should be able to fire an already loaded and aimed ballista.

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    \$\begingroup\$ medievallifestyle.com/siege-engines/ballista.html claims "A large ballista could launch an imposing javelin weighing up to 10 pounds", which seems like a perfect fit for Mage Hand, especially if it is being fired from a smaller ballista. \$\endgroup\$ – Dragon Aug 27 '20 at 4:36
  • \$\begingroup\$ Huh. I would have expected the bolt to be heavier. Guess loading might be good after all. \$\endgroup\$ – shhalahr Aug 27 '20 at 13:56
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    \$\begingroup\$ 60 pounds for the largest Roman shots: britannica.com/technology/ballista . I don't know of a reliable source for how light they could get, if there even was a cut-off from arbalest. Wikipedia mentions 26 - 78 kg. \$\endgroup\$ – Tommi Aug 27 '20 at 14:09
  • \$\begingroup\$ The wikipedia ammo weight of 57 lb. belongs to a stone shot for a 1 talent caliber ballista (see how huge that ballista is in comparison to the legionary to its side en.wikipedia.org/wiki/Ballista#/media/File:Bal_BBC1.jpg). The question implies to ask about bolts (some bigger version of crossbow bolts up to spear like size) fired from a smaller ballista, more like a scorpio (en.wikipedia.org/wiki/Scorpio_(weapon)). \$\endgroup\$ – Luke T. Deril Mar 12 at 12:30
  • \$\begingroup\$ In fact, the power to operate such a smaller version can be done one handed for using the winch, placing the bolt, and pulling the trigger (more like a lever or pulling a rope with a hook than a modern pistol trigger). Aiming may require the assistance of divination magic or a telepathic link with a familiar in position, to make use of the "targeting system" of the weapon (almost using end and tip of the bolt as notch and bead sights). \$\endgroup\$ – Luke T. Deril Mar 12 at 12:30
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The Mage Hand description says

The hand can't attack, activate magic items, or carry more than 10 pounds.

Loading a ballista requires some force, which I am pretty sure that is greater than 10 pounds (considering it as weight-force). Aiming requires some force too, even if some leverage may ease this.

I think that by using Mage Hand one can "fire" a ballista, since it is not a magic item, provided that the lever (or whatever activates the ballista) does not put too much resistance, i.e. a force greater than the weight-force of 10 pounds.

Hence, I think that Mage Hand allows to "fire" a ballista (depending on the DM), but does not allow to load it and to aim.

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