After reading the optional rule of Hitting Cover, it strikes me that the rule is flawed.
The Hitting Cover variant rule states (DMG p. 272):
When a ranged attack misses a target that has cover, you can use this optional rule to determine whether the cover was struck by the attack.
First, determine whether the attack roll would have hit the protected target without the cover. If the attack roll falls within a range low enough to miss the target but high enough to strike the target if there had been no cover, the object used for cover is struck. If a creature is providing cover for the missed creature and the attack roll exceeds the AC of the covering creature, the covering creature is hit.
So imagine that I want to hit a target, behind another creature. For simplicity, we will refer the target as "target", and the creature who is providing cover to the target as "creature".
The target has an AC of 16, and receives a bonus of +2 due to half cover from the creature.
The creature has an AC of 18.
I roll to attack the target, and roll a 17.
I would hit the target without cover, but I do not hit the target with cover: so I hit the creature that is used for cover. (16 < 17 < 18)
But since my attack roll is lower that the creature's AC (17 < 18), I miss him.
Is it not possible at all to hit the creature, as long as the creature providing cover has an AC that is 2 higher than the target's AC?