Can archers bypass partial cover by arcing their shot?

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

If part of the target is visible 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 the archers are using whatever form of fire is most effective for the situation and that RAW (those adjusting for cover) apply.

However, it may be the case that the archers wish to fire on targets which are behind a wall which is 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 indirect fire as a non-RAW option...

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 (emphases 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. Since the ballista (DMG 255) is a direct-fire siege weapon, we can compare its features to those of a mangonel and trebuchet to see what effects, RAW, indirect fire imposes. That can then give us a baseline for how to modify archer fire while remaining within the spirit of RAW.

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 that it is harder to hit a target with indirect fire. Archers firing indirectly should be assigned a penalty to hit.

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 in part because of the technological limitations of the weapon, but also because the high arc required to hit such a close target 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 describing projectile motion, but the siege weapon descriptions have a minimum range of 1/3rd to 1/5th normal range. A similar minimum for the archers would be appropriate.

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 that could intercept a high arc. Again, you can calculate firing arcs, or you can adopt a simplistic rule such as 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 are typically buildings, defensive fortifications, and other siege weapons, none of which move during the time frame of their firing. Since their target isn't moving, it is not as important whether they can see the target or not on an ongoing basis. If the targets of the archers are both behind total cover and moving, I would suggest using the standard penalty of disadvantage on attacks for opponents you cannot see.

Finally, 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.