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There are two basic ways an archer fires at a target. In close quarters engagements, archers (and anyone using a projectile weapon) would likely use "direct fire", ie. fire at an angle nearly parallel to the ground. At longer distances and especially when targets are hiding behind terrain and walls, archers instead use "indirect fire", ie. firing at angle greater than 45 degrees, in order to lob arrows over and behind cover.

Imagine an archer firing at a range of 100 feet on a creature using 5-foot tall wall for cover.

The rules for cover on a grid state the following:

Choose a corner of the attacker’s space or the point of origin of an area of effect. Then trace imaginary lines from that corner to every corner of any one square the target occupies. If one or two of those lines are blocked by an obstacle (including another creature), the target has half cover. If three or four of those lines are blocked but the attack can still reach the target (such as when the target is behind an arrow slit), the target has three-quarters cover.

Using these rules, it's easy to see how the wall could provide half or three-quarters cover against direct fire. The trajectory of the arrow will always intersect with the wall, and if enough lines from the archer intersect with the wall, then partial cover is granted. This is consistent with a physical understanding of the scenario, because the arrows will follow a nearly straight line from the archer to their target.

But what if the archer chooses to fire indirectly at their target? In the physical world a wall would provide no cover against an attack that falls from above. Drawing lines from the archer, however, results in the same result as direct fire, granting partial cover in a way which is inconsistent with reality.

Are there any rules that would allow the archer to use indirect fire to bypass partial cover?

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Yes, and it's hard, and that's why there's a cover penalty.

We generally assume that the character already knows the basic techniques for using their weapon, and thus gets no additional benefit from the player saying they intend to use it correctly. You're getting a proficiency bonus because you're proficient.

And if the enemy is behind cover such that it makes sense to try to arc a shot over, then of course you'll do that rather than thunk an arrow uselessly against the wall. However, hitting someone with a shot like that is very hard. The system already has a way to represent that extra difficulty, which is a bonus to the target's AC.

Now, there is potentially an important difference between the two techniques: if you try to shoot through a gap in the wall and miss, you hit the wall, but if you arc your shot over the wall and miss, your shot likely lands somewhere behind the wall. The usual assumption in D&D is that it doesn't matter--missed shots just miss everything. In a case where it does matter (like if there's a crowd of people on the other side of the wall) the group would need to agree on some house rule to decide who, if anyone, gets hit.

Note also that if they're completely behind the obstacle, you probably can't see them, so you'd have disadvantage on the attack roll.

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    \$\begingroup\$ While at first glance this seems like a good interpretation, this ignores the fact that all shots beyond a certain range require indirect fire. There's no partial cover penalty for firing a shot at a creature, say, 200 feet away (besides disadvantage if it falls in the weapon's long range) so it doesn't seem like this isn't actually a consideration of the rules for Partial Cover. \$\endgroup\$
    – Andrendire
    Dec 28 '20 at 2:28
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    \$\begingroup\$ Simulationist answer: the cover bonus to AC handles things such as the attacker not being able to see the target's shifting weight and such, so the target can more readily shift enough to avoid the projectile (or have it bounce harmlessly off their armor, or...). \$\endgroup\$
    – minnmass
    Dec 28 '20 at 4:54
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    \$\begingroup\$ @Andrendire DnD is not a 1:1 simulation of real-world physics. It is assumed that if something is in the way of your arrow hitting the target, then it still can hit if it is possible for you to aim it to avoid that cover, and the AC penalty simulates that regardless of the actual range. Whether doing so means you have to aim for a smaller space or adjust your aim to a specific arc beyond normal, it is still trying to simulate the increased difficulty of landing the shot. \$\endgroup\$
    – Zibbobz
    Dec 28 '20 at 13:55
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    \$\begingroup\$ @Andrendire A shot at any range will drop some distance in flight (caveat: those tests were done with a modern hunting crossbow, which delivers less energy than a medieval war bow). An archer familiar with their weapon will know to adjust for this. But there's a big difference between aiming correctly for the range of the shot and trying to shoot over an obstacle. Otherwise, nobody would bother building walled castles. \$\endgroup\$
    – Mark Wells
    Dec 28 '20 at 18:14
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    \$\begingroup\$ @Andrendire If we're standing 500 feet apart, equidistant from a 4' wall, and you're trying to shoot me with a bow, I would not consider the wall to be cover at all, because it does not project into the path of the shot you're trying to take. The D&D combat rules do not contemplate this specific possibility because (1) they're designed for combat at much closer ranges and (2) it's kind of a niche case. The game is not a physics simulator, but you can certainly simulate physics when playing it if that's what you're into. \$\endgroup\$
    – Mark Wells
    Dec 28 '20 at 20:56
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DnD is not a physics simulator

There are, as far as I'm aware, no rules for arcing projectiles so that they avoid cover. The rules only describe directly targeting somebody and having the projectile follow a completely straight line.

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    \$\begingroup\$ On top of that, firing a projectile in such a matter would probably be done with disadvantage since it would be a much harder shot to make. \$\endgroup\$ Dec 27 '20 at 23:57
  • \$\begingroup\$ If you are looking for a narrative justification, low walls would obscure the archer's vision and ability to aim their shot. Also, lobbed arrows don't come from 90°, or even 45° at that range. The obstacle would even still provide some cover from any targeted fire \$\endgroup\$
    – Alex F
    Dec 27 '20 at 23:58
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If part of the target is in view above the wall, the existing rules for half or three-quarters cover address the situation adequately, without having to complicate the matter by deciding whether the archers would be using direct or indirect fire. You can assume that they are using whatever is appropriate for the situation and that RAW apply.

However, it may be the case that the archers wish to fire on targets which are behind a wall higher than the targets. That is, there may be targets which, in two dimensions, are behind complete cover and thus are not targetable RAW, but for whom indirect fire is a realistic option.

If you wish to permit that as an option beyond RAW...

You can use siege weapons as a model

While combat in three dimensions is largely unsupported in 5e, and archers using indirect fire is not explicitly mentioned, there are two siege weapons that explicitly use indirect fire. Rules that already exist can guide you if you as a DM decide to make this a homebrew option.

From Siege Equipment, DMG 255-256 (emphasis mine)

A mangonel is a type of catapult that hurls heavy projectiles in a high arc. This payload can hit targets behind cover. Before the mangonel can be fired, it must be loaded and aimed. It takes two actions to load the weapon, two actions to aim it, and one action to fire it. A mangonel typically hurls a heavy stone, although it can hurl other kinds of projectiles, with different effects.
Mangonel Stone. Ranged Weapon Attack: +5 to hit, range 200/800 ft. (can't hit targets within 60 feet of it), one target. Hit: 27 (5d10) bludgeoning damage.

A trebuchet is a powerful catapult that throws its payload in a high arc, so it can hit targets behind cover. Before the trebuchet can be fired , it must be loaded and aimed. It takes two actions to load the weapon, two actions to aim it, and one action to fire it. A trebuchet typically hurls a heavy stone. However, it can launch other kinds of projectiles, such as barrels of oil or sewage, with different effects.
Trebuchet Stone. Ranged Weapon Attack: +5 to hit, range 300/1,200 ft. (can't hit targets within 60 feet of it) one target. Hit: 44 (8d10) bludgeoning damage.

From these descriptions it is clear that indirect fire is a 'thing' in 5e. Comparing these siege weapons to a direct-fire siege weapon (the ballista, DMG 255) will give us a basis for modifying the attacks of your archers.

First, note that while both of the indirect-fire weapons are at +5 to hit, the ballista is at +6 (and the ram at +8). It seems reasonable to assign a penalty to hit for an archer's indirect fire, or perhaps a reduction in the amount of proficiency bonus that can be used.

Second, both of the indirect fire weapons have minimum ranges of 60 feet, while the ballista does not have a minimum range. Presumably this is because the high arc required to hit a target any closer than that would make the resultant shot so inaccurate as to not be worth it (and would possibly include the weapon itself in the area potentially receiving fire). You could get pedantic with arcs and projectile motion, or you could borrow from the two siege weapon descriptions and assign a minimum value as around 1/3rd to 1/5th short range.

Third, "Siege weapons are designed to assail castles and other walled fortifications." (DMG 255) Implicit in this is that they are operating outside - with no ceiling to intercept a firing arc. Again, you can calculate firing arcs, or you can adopt a simplistic rule that when indoors or underground your archers can't fire on anything further from them than twice the height of the ceiling.

Fourth, as siege weapons, their targets typically are buildings, defensive fortifications, and other siege weapons, none of which move during the time frame of firing. Since their target isn't moving, it is not as important whether they can see the target or not. For archers attempting indirect fire at targets behind cover, I would suggest using the standard penalty of disadvantage on attacks for opponents you cannot see.

Finally, realize that because of the angle of the descending arc, indirect fire won't be able to easily hit thin creatures that are deliberately snug up against the far side of a wall. It is still appropriate to give targets an AC bonus due to cover if they are using this to their advantage.

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  • \$\begingroup\$ Very interesting find! I suppose that introducing a penalty to hit is effectively the same thing as a partial cover bonus, but it's interesting that indirect fire is at least considered in the DMG. \$\endgroup\$
    – Andrendire
    Dec 28 '20 at 2:25

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