I was thinking about what the possible uses of the cantrip shape water could be, and I realized that it might technically be possible for you to make infinite ice arrows.
Would this work? If so, what damage would something like this do?
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If we want to get all science-y about it, arrows have to be flexible. It's known as the "archer's paradox" -- an arrow goes where you aim it at full-draw even though it seems like it would have to go through the bow's body to do that. The reason is the arrow flexes back and forth as it flies just after release, rather like a swimming fish, and that's caused by the arrow actually bending around the bow; you can see this in slow-motion videos of archery (or in Disney's Brave).
Selecting the proper arrow stiffness, or spine, is critical to getting good accuracy out of it. You have to match the arrow's spine to the draw weight of the bow you're shooting with, among other things -- and this is in fact one of the most complicated parts of archery. There are dozens of variables involved in shooting with a compound bow that all affect how much spine you want. Too much spine or too little can both introduce random inaccuracy into the shot. (And of course an ice arrow would likely shatter under the launch forces involved in firing it.)
Since when do we let real-world physics tell us what to do? This is fantasy! We have magic! There's nothing in the rules of shape water that would stop us from making arrows from ice.
As a DM, I'd probably allow this because it's clever, and I like clever, but on the other hand a cantrip really shouldn't be a replacement for the time-consuming skilled craft of arrow-making (properly called fletching). This is a good emergency measure, but shouldn't be your everyday source of ammo. I'd probably apply disadvantage on the attack rolls, as the arrows aren't properly flexible, aren't as hard as a steel arrowhead, and are potentially misshapen (due to production flaws, melting, and so on). I'd also rule they can't be recovered because they shatter on impact.
Alternatively, a DM could treat the arrow as a generic improvised weapon, but that gets a little weird because generic improvised weapons can be thrown but there isn't a rule for using them as ammunition, so it's getting pretty far into houserule territory.
The text of the spell reads, in part (emphasis mine):
You choose an area of water that you can see within range and that fits within a 5-foot cube. You manipulate it in one of the following ways:
- You cause the water to form into simple shapes and animate at your direction. This change lasts for 1 hour.
- You freeze the water, provided that there are no creatures in it. The water unfreezes in 1 hour.
If you cast this spell multiple times, you can have no more than two of its non-instantaneous effects active at a time.
This seems to be within the bounds of two separate castings if and only if you consider an arrow (presumably with sharpened head and something that serves as fletching on the shaft) to be a "simple" shape. That's GM call number one.
GM call number two is whether an archer can usefully shoot an arrow made out of a slippery material like a shaft of ice.
GM call number three is whether an arrow made of ice (probably made by someone not an expert archer) is really useful as a weapon.
In my experience, three sequential judgment calls like that is a "Maybe, but probably not," because you only need to fail one of them before the whole chain of logic falls apart.
I, as a GM, would be caught between my desire to reward player creativity if they're using this to get out of a jam, vs my unwillingness to see the players abuse this by resorting to simple cantrips as a way of never having to buy ammunition again because cantrips are cheaper. So I'd probably let it work, but make it substantially worse than using normal ammunition, i.e., with a penalty to hit and/or to damage, or application of disadvantage.
The two clauses of Shape Water that could in theory contribute to an effect like this are these two applications:
You cause the water to form into simple shapes and animate at your direction. This change lasts for 1 hour.
You change the water’s color or opacity. The water must be changed in the same way throughout. This change lasts for 1 hour.
You freeze the water, provided that there are no creatures in it. The water unfreezes in 1 hour.
Both effects must fit in a 5ft cube. Can a "Simple Shape" be the shape of an arrow? It doesn't say. Can you freeze the water into an arrow shape? It doesn't say.
The rules concerning Improvised Weapons state the following:
Often, an Improvised Weapon is similar to an actual weapon and can be treated as such. For example, a table leg is akin to a club. At the GM’s option, a character proficient with a weapon can use a similar object as if it were that weapon and use his or her Proficiency Bonus.
However, arrows are not weapons. To quote the rules of the Ammunition keyword:
You can use a weapon that has the Ammunition property to make a ranged Attack only if you have Ammunition to fire from the weapon. Each time you Attack with the weapon, you expend one piece of Ammunition.
Arrows are ammunition, and they do not have any influence on the damage you deal with your ranged weapon unless it is a magic item that specifically states otherwise. If you make an attack with a Longbow, and fire an arrow from it, unless stated otherwise by a magic item or ability you have, it will deal piercing damage equal to 1d8+Dexterity modifier. You can fire the same arrow from a Shortbow, and it will now deal 1d6+Dexterity modifier damage.
First of all, you only get one of the possible effects per casting of shape water. So you can only either freeze or shape the water with a single casting. So, we need to cast at least twice for each set of arrows. That's 12 seconds, and the items only last an hour.
Now, you can only affect water that is inside a 5-foot cube. That means, we only can have a limited number of arrows made in a single cube. But how much? That depends on the bow. Let's see... an 80 lbs English Long Bow with 30-inch draw length is supposed to have an 80-85 Spine value, which is material dependant value from which one can look up the thickness of the appropriate arrows. What does it stand for? Well, I'm glad you ask: Spine is how much it needs to bend to be perfect for the bow and not snap on firing. With some known kinds of Material, the formula is pretty much a Variable * Diameter = Spine.
Now, that variable for ice is 0, and as a result, it should never be an arrow that can be shot. But let's ignore that for the sake of coolness and instead assume our arrows will instead have the measurements that the vast majority of the 3500 Mary Rose arrows had: 12 mm (½″) tapering to 10 mm (⅜″) at the nock, 790 mm length (30' draw length) and feathers reaching about 40 mm from the shaft for 7 1/2″.
This results in the above packing without having any of the feathers touch each other. According to the construction, there's about 47 mm of air in between each shaft's long axis and it's closest neighbor's short axis, and 80 mm between each arrow and its second closest neighbor. Or in other terms: a little above 700 arrows can be pushed into a 5x5 board without the feathers touching and all in the same direction. With a little bit of clever stacking, we could arrange for three sets of arrows getting stored at a slight angle without any arrow feather touching any other shaft, so our total maximum storage capacity in one 5ft cube is 2100 arrows. And it would also weigh in the order of one metric ton.
Now, if you can have more depends on a tiny interpretation: Does the water freeing count as instantanous effect or do you need to keep it up? If you can just have it stay, you can create quite some more, otherwise those 2100 arrows are the limit.
Under the long interpretation the production is 2100 arrows per 12 seconds. 3600 seconds per hour, so 300 casting cycles before the first arrow starts to melt. That's 630000 arrows weighing several hundred tons.
While that is technically not infinite, in both cases that is more than enough arrows to ever come up - as long as you have 2 rounds within the last hour, you could always have a full quiver.
I would let you do it exactly once, and then point up to the one point where I went away from reality: your ice arrow shatters in your face and you are lucky that you didn't ruin your bow or take damage for such lunacy.
However, if you make crossbow ammunition, that doesn't need to flex, I'd let you do that as much as you want - but handle the ammunition as vastly inferior to any proper bolts - cutting damage as I'd deem appropriate.
To construct a functional ice arrow you'd need to be able to fabricate it using something more complex such as single-crystal microfibres of ice. This degree of fine control of the material is beyond the scope of the "simple shapes" that shape water can do.