To put it simply, let's say that a missile attack "misfires"—that is, fires toward an unintended target—when the attack check comes out under the defendant's unarmored AC.* In this case, each creature immediately adjacent to the intended target who is also within range of the missile attack will need to make a saving throw adding their Armor Bonus (equal to their AC-10) against the missile's attack roll. On a failure, they suffer the attack's normal damage.
And with that said, let me try and write through some of my reasoning.
In Shadowdark RPG a character's unarmored AC is 10 + their Dexterity modifier. Although it is not said so directly, we can suppose that this implies that 10 is the lowest an assailant needs to roll in order to properly strike their defendant. To explain, let's say that Dagmar is attacking an unarmored bandit who has a Dex modifier of +3: On a roll of 1–9, Dagmar swings poorly and simply misses the bandit—it would have gone this way even if the bandit was not particularly nimble. On a roll of 10–12, however, the bandit must have dodged, as this is the window in which their higher Dexterity modifier comes into play.**
The Shadowdark rules define Leather Armor as giving "AC 11 + DEX mod," however it may be more useful to phrase it as +1 to unarmored AC. This way we can tell that if Dagmar's bandit is wearing Leather Armor, the attack roll of 13, which would have landed when they were unarmored, now misses specificly because it is rebuffed by the armor. It naturally follows that Chainmail armor works the same but with +3, and the shield's +2 sits neatly on top of it all.
Breaking it all down this way we can see the narrative thresholds of the Armor Class system. Dagmar's Dexterity modifier is +2, and he wears Chainmail Armor and uses a shield, netting AC 17: An attack roll of . . .
And now my rules for misfires hopefully make sense. A failed bow attack that rolled above the defendant's Unarmored AC has been deflected by either their armor or shield, but a lower roll means that the arrow makes no contact with the target, and can therefor plausably hit someone else. (While a defendant wearing Platemail is simply letting arrows bounce off of them.) The purpose of Armor Bonus is then to calculate a defendant's actual degree of protection, distinct from an assailant's chance of failure. AC-10 gives you all the skill and armor bonuses at their disposal.
As it stands, my intention is to determine at the table in which order potential misfire targets are most likely to be hit, and have them all roll in order, stopping if/when someone fails their save. If it works, this system should make misfires something which everyone will hold as a concern, without making them incredibly common. I also think that this will strike a good balance between realist plausability, and overly complex simulationist rules. Giving higher attack rolls a higher chance of hitting misfire targets is important, I feel, because it demonostrates that higher attack rolls still mean more effective attacks.***
If you wanted to trade simplicity for further realism, though, you could also try determining first which way the arrow misfires in order to see who the potential targets are. One way you could do this is by using evens and odds to determine right or left on attack rolls under 9 which have missed entirely (if 1 means that the assailant fails to loose an arrow entirely, then there will be equal chance of even or odd in this range), and then saying that dodged missiles hit characters behind the target. In this system, I would consider also ditching the saving throw entirely, and simply checking the attack roll against the misfire target's AB (or maybe something like AB+5), as so vastly limiting the quantity of characters making saving throws could potentially make misfires too rare.
* Or simply 10, if the defendant is wearing Platemail, and is therefor not using their Dexterity modifier in evading attacks.
** If a character has a negative Dexterity modifier, then we can suppose that a roll of 9 (or perhaps lower) should have missed, but that the clumsy defendant stumbles into the way.
*** The familiar practice of having critical failures result in an ally next to the target getting hit has never felt right to me, as it interprets the lowest possible attack roll as only a near miss on the battlefield.
Contact me at LechterDoily@gmail.com!