Shooting into melee combat

When shooting a ranged attack into melee combat, the target gets +4 to their AC due to the difficulty of shooting through the mess of foes.

If the attack misses, then the attack will instead hit somebody who has less AC than the attack roll.

Example:

Fighter is battling a demon. The demon has an AC of 20. The Fighter has an AC of 17.

The party Ranger tries to help and shoots into the melee, targeting the demon. The demon normally has 20 AC, but for the purposes of this attack, has 24 AC. This is because the Ranger needs to take extra care to hit the right target.

The Ranger rolls their attack and gets a 21. 21 is too low to beat 24, so the attack misses the demon. However, because 21 is higher than the Fighter's AC of 17, it hits the Fighter instead.

If it's lower than the Fighter's AC as well, then the attack misses and hits nobody.