In a typical D&D game your to-hit roll generally has four results, a miss so bad that it causes a fumble (rolling a 1), a hit so good you score extra damage (rolling a 20), a miss (rolling below the target AC), or a hit (rolling at/above the target AC). Outside of success and failure, there is little consideration on what the target’s AC is versus what you roll.

There has always been a constant debate on critical hits on if you should give max damage, an extra damage roll, or double damage.

I was wondering why not add a point of damage for every number above the target AC that you roll? This way a 20 isn’t a special critical hit, it’s just giving you the max bonus to your hit.

As monster AC increases the benefit of this goes way down (a roll of 20 on an AC17 monster is only giving +3 whereas that 20 on an AC13 monster is giving +7) but I think it’s reasonable to assume that the higher the AC is simply harder to hurt.

This is something I’m still bouncing around in my mind, would love to hear what others think. I’ve also been wondering what if we dropped the damage roll altogether and just used the above-described bonus as the damage dealt. Consider this a work-in-progress idea, like most things you’ll see posted here.

There is a very good skirmish game called 'Rangers of Shadowdeep' that uses something similar to this on an opposed roll basis. If my memory is correct both players roll a d20 and the damage is the difference. between the two after modifiers.

It's worth checking out, though IMO systems like these become a bit too complex because of the numbers involved.

The system that I very much enjoy is the 'Warmahcine' system (another wargame), where you have a Defence (How hard to hit), and an Armour (how tough) stat, and when you roll damage all weapons rolls 2d6 and it's the amount over the armour that is inflicted. The numbers are a bit more manageable in this kind of system.

My slight alteration to this rule is to have the number of success levels on an attack roll equate to the minimum number that can be rolled on the damage dice. For example, if you require a 13 to hit, but roll a 17, that's 4 success levels, so when you roll your D8 long sword damage you treat a result of 1-4 as a 4. This is capped at the weapons maximum damage, so 5 success levels with a dagger strike would still only result in 4 damage. I just got fed up with rolling a 19 to hit and thinking wow, what an awesome strike, to then roll a 1 on the damage dice. It didn't seem right after such a "solid" hit.