I’m trying to find if the D&D rules support players maneuvering to fire ranged weapons down the long axis of an enemy formation.  I’m a new player but I’m always impressed with how detailed the rules are and how much people know about the game - I’d be very surprised if there wasn’t some sort of bonus for a player who put themselves into a smart firing position where if they missed the front target they had a good chance of hitting secondary targets in the background.  

Not as written, but a DM might give you advantage on the attack for smart positioning.

No, I don't think the rules cover this. If you miss your target, you don't get a chance to hit another target.

The only instance in which this matters is the Catapult spell. That spell lets you hurl an object on a predetermined line up to 90ft long. The object travels the length of that line and then falls to the ground at the end, presumably straight down without preserving any of the momentum (it's magic, don't question the physics). If it encounters an obstacle along the way the object smashes into it, doing damage to both the object and the obstacle. If the obstacle happens to be a creature, it gets the chance to make a saving throw to avoid being hit and allowing the object to continue hurtling, if multiple creatures are along that line, then they all have a chance to be hit, as long as the creature in front of them made their save to avoid it.

Yea I had an argument with a player about this the other day. He was trying to shoot an invisible monster down a narrow (10'-15') corridor. He made this exact same question. I said he cannot unless he can pinpoint where on the map the enemy is (I had the location in my mind) since the game does not support that unless for specific spells, like catapult. He would be shooting with disadvantage regardless of whether he found the location of not (blind target rules). I did point out that even if one player was using another player as cover (gaining +2 to AC) and happened to get hit within those 2 AC points neither of us (he DM'd last campaign) would cause that attack to hit the player being used as cover).

I ended up letting him take the attack with disadvantage and would have modified any bonuses based on which square he picked on the grid and how close it was to where the invisible creature is. I also allowed this since the turn before the sorcerer used Aganazar's scorcher and set the entire column aflame so he also used the fact that the floor is alight, there should be a singe mark or at the very least they would've heard a noise as the monster took damage. Fair enough

I appreciate you all taking the time to respond. Our DM is very clever, I’m sure he can figure a way to do this fairly even if the rules don’t explicitly describe it.

much thanks 

Quote from Noxx89 >>

Yea I had an argument with a player about this the other day. He was trying to shoot an invisible monster down a narrow (10'-15') corridor. He made this exact same question. I said he cannot unless he can pinpoint where on the map the enemy is (I had the location in my mind) since the game does not support that unless for specific spells, like catapult. He would be shooting with disadvantage regardless of whether he found the location of not (blind target rules). I did point out that even if one player was using another player as cover (gaining +2 to AC) and happened to get hit within those 2 AC points neither of us (he DM'd last campaign) would cause that attack to hit the player being used as cover).

I ended up letting him take the attack with disadvantage and would have modified any bonuses based on which square he picked on the grid and how close it was to where the invisible creature is. I also allowed this since the turn before the sorcerer used Aganazar's scorcher and set the entire column aflame so he also used the fact that the floor is alight, there should be a singe mark or at the very least they would've heard a noise as the monster took damage. Fair enough

I'd like to point out that a creature being invisible does not make it so that other creatures don't know where they are.  Only taking the Hide action makes your location unknown to present creatures; being invisible just gives you the option to Hide anywhere, but unless they actually take the action to become hidden, their location is known.  Additionally, even if they do Hide, the location that they did so in is known, so unless they Hide and then move it's kind of pointless.

That said, giving a ranged attack the potential to hit any creature in a line if it misses those in front would be quite OP.   That would basically be giving you several chances to land your attack, when you should only get 1.  Spells like Lightning Bolt are the ideal for this situation.

Nothing as written, but I could imagine an easy enough houserule where you fire at the first target, and if miss, check the same roll against the target in a line directly behind with a +2 AC bonus from half cover, and if miss, check the next in a line directly behind with a +5 bonus from 3/4 cover, and if miss, stop because they would have full cover from then out. Note that it doesn't really make it easier to hit creatures in formation like OP is looking for, but at least somewhat addresses the possibility of missing your target and hitting whatever is behind it. 

But even with this rule, I would not be permitting seperate rolls against each target, and I would not be very flexible on "straight line," i'd be wanting a ruler-edge straight line of targets lined up to allow any missed shot second chance checks. 

I would say enfilade fire is most effective against formations and since 5E is broadly focused on individual combatants it is worth considering that an attack roll vs. AC doesn't represent how the attack missed, only that it did. Was the bow aimed too high, too low, too far to the left or the right? Was the arrow deflected by a metal shield or stuck in a wooden one, graze an ear and plunge into the enemy behind or was just warped and wobble off? Because an attack roll has missed doesn't necessarily mean that it threatens something else beyond the original target. 

Acme is correct. An attack could 'miss' for any number of reasons many of which would eliminate the possibility to hit a target further downrange.

Re: taking cover behind another for +2 AC might make me have the cover character get hit instead. It's situational, and I would totally do it in a hostage situation.

I feel there are enough abilities in the game which enable this kind of thing - Arcane Archer's Curving Shot comes to mind - that allowing it could step on some character's toes.

As acme and wysperra are saying, the basic abstraction in D&D combat isn’t that you roll to hit, it’s that you roll to hit for damage. A miss doesn’t necessarily mean a complete whiff, it can mean the arrow glanced off the target’s armor. I wouldn’t get too deep into what happens after you don’t hit. 

A different house rule might be, if you’re just aiming for the formation in general, but not too concerned about which target, to give the group as a whole an AC, and fire away. That could mean some extra bookkeeping for the DM to track if random soldier 6 or 9 was the one who took damage, but it covers the flavor well, imo. Maybe even just use minion rules and say if you hit you drop one of the targets, and avoid that. 

On the flip side, I've actually had to explain to new players that AC isn't purely your armor, but - as has been said - can encompass all reasons an attack may fail.  Basically explaining that while a tank may have really high AC, without their armor they only have 10 (or less,) whereas without their armor a Dex-based character can still have decent AC because of their ability to dodge.

Reminds me of a recent situation: I had a player that was trapped inside of a Shambling Mound and tried to attack it.  Since being engulfed by the creature imposes both the restrained and blinded conditions, I of course told him to roll with disadvantage.  "How can I miss something I'm literally inside of?"  He asked.  I explained that it's not about being able to hit it; indeed, his weapon is most likely already in contact with the thing.  It's about getting enough momentum/the proper trajectory/whatever to actually cause any damage.

Enfilade fire also tends to assume a high rate of fire weapon - either a semi-automatic combat rifle or a machine gun.  When spraying down the long axis of a formation, such fire can be devastating.  Bowmen in close range of the formation are distinctly less advantageous, as anyone who doesn't drop from the initial shot can charge the archer.  if he's far enough away that they can't, his fire is also likely arcing high, and plunging fire doesn't care about enfilade at all.

Maneuvering to deny an enemy their cover bonuses is smart, but 5e is specifically very lean on the ranged combat rules.  The game assumes most entities will be engaging in melee combat, and that distance fighters are the exception, not the rule.  Despite the fact that typical party comp is distinctly not that way.  Silly game designers.

If a party is fighting a mob (of goblins or orcs for example) and an archer has a "to hit" number under 20, and the roll is equal to one under the "to hit" value, I would be inclined to pick a target downsteam and make it a hit on that target.  If the "to hit" number was under 13, I might be inclined to consider going two values under the "to hit" number for "downstream hits", provided there were enough targets to justify it.  I guess there are three possible outcomes in general … A damage dealing hit, a "plink" off the target, or a whiff.  Any whiff could hit a downstream target if there is one.

The "Rule of Cool" (or is that the Rule of Cule?) would say a whiff-hit is a good idea, in my view.