Making D&D combat realistic
This post comes out of thoughts in this thread over on theRPGsite. Basically, the question was which RPGs offer a realistic model of combat, and based on the way the question was phrased, I took this to mean that the model produces results which, when interpreted at the level of abstraction of the system, are commensurate with the inputs.

I wrote,

Well, in combat between humans, without magic [...] if you take the relative experience, armor, and weaponry of combatants into account as inputs, D&D will yield a result that's roughly commensurate given the abstract nature of hit points. In a melee you will end up with one man dead, surrendered, or fled, and the other victorious although possibly weakened somewhat. There are some areas I would call into question:

• Shields are relatively undervalued in terms of their overall contribution to survival. The relative significance of other types of armor compared to each other could also be questioned, but it's not egregious.

• The value of experience also seems to be distorted. At the low end, the likelihood that a trained but "inexperienced" (1st-level) warrior will die in a stand-up fight with an untrained civilian seems rather high. At the high end, it is possible for a fighter to outclass even an experienced opponent to such a degree that there's effectively no doubt as to the outcome.

[...]

Another quibble that's not in quite the same class is the handling of missile weapons. While it's easy to rationalize the loss of hit points by the victor in melee, it's pretty hard to explain the ablative "weakening" of a combatant by arrows that narrowly miss or glance off armor (i.e., cause HP loss but don't kill). And this has another significant effect on the outcome in that archers expend arrows as they attack.

Nevertheless you could still argue that D&D is more realistic than many other games, since those are far more likely to produce outcomes where combatants unambiguously suffer major wounds yet keep fighting and defeat their opponents. [...] I think that most actual wounds inflicted by melee weapons will either be fairly insignificant or will effectively take you out of the fight altogether.

Later I wrote that the main obstacle to achieving "verisimilitude" in D&D was the use of abstract hit points to represent "survivability".

So this has me thinking what might be the minimum changes I could apply to the basic D&D combat system (I mean pre-AD&D 2e) to improve its verisimilitude and realism. These are just some ideas, written up in the spirit of keeping the game simple, and also without too much concern for "fun". What I mean by that is, I think most RPG players dislike high lethality, especially for "high level" characters. These rules changes by contrast make the game a lot less safe; they might be more acceptable as a set of skirmish rules than as an RPG module.

#1: Instead of lowering AC by 1, shields lower AC by 5. This is a rough value, and can also be modified to represent large vs. small shields.

#2: Characters start at 2nd level and cap out at 5th level. Again, this is a rough limit. Furthermore, if you also follow my recommendation of doing away with escalating hit dice/level, it may not be necessary to implement this change. If you do still use hit dice/level, I'd recommend giving max HP (+ Con bonus) for the first die, then average HP for each subsequent die (with or without Con bonus). Otherwise, if you want some randomness, then consider a method that has a lower variance than just rolling a single die. However, if you want to be really hardcore, go ahead--the fact a some fifth-level fighter could end up with 7 HP simply reflects the fact that life is unfair and "living up to your potential" might still mean you suck.

#3: Probably not necessary if you use #4, but an alternative missile weapon system is needed. I can't be bothered to develop the details right now; the basic idea is that if you are hit by a missile, you will be at least significantly wounded regardless of your hit points. It could be a chart per weapon, or some kind of saving throw after you compare rolled damage vs. Constitution, and results could be expressed in terms of % of starting HP lost, or direct description ("no effect", "incapacitated", "dead").

#4: This is really the most interesting bit...it may also be stolen from somewhere (Pendragon?). The basic idea is that survival is achieved by not being hit, and in a normal combat, one maneuvers to an opponent's disadvantage such that, when all is said and done, striking your opponent generally means that you are not struck back. Mechanically, we do it this way: do not add hit dice when going up levels. Then, when in combat, declare a target and roll a d20. This die is both your attack and defense. In order to hit, you must roll greater than the number needed to hit the target's AC (as in the standard combat tables, or via THAC0 calculation for you newfangled types). But if two people are facing each other and both hit, only the one who rolls lower actually hits. (If they roll equal, both hit.)

("Rolls lower" seems right, but "rolls higher" might also work.)

Note that if you don't want to attack, you don't have to; your die is just your defense in that case; your opponent must "beat" your die (roll lower--or higher) while also beating your AC. I haven't given much thought as to whether an "all out dodge/parry/block" needs to added; in principle, under the model I propose, you could argue that if you aren't threatening your opponent, he will have an easier time striking you. But conversely you could argue that people sometimes fight without regard to preserving themselves, and if you're up against someone like that, you'll be less likely to be hit if you go into full defense rather than threatening them.

It could even be that you might get a useful effect from allowing players to declare before rolling that they're taking "high hits" or "low hits"--meaning that they'll hit when rolling higher than the opponent (provided they also hit on the combat table), or they'll hit when rolling lower than the opponent (provided they also hit on the combat table). I haven't thought much about this.

Another possible improvement would be to implement a 3e-like concept of "base combat bonus" in lieu of the columns in the combat tables or increasing THAC0 as you go up levels. The main benefit here would be that you'd apply the bonus to your combat die (as a subtraction on defense, if the opponent is trying to roll under, but as an addition for all other cases including attack). The purpose of this wouldn't be so much to move to a 3e-like system of target numbers and "better armor means higher AC"--although you could rejigger the math in that direction, to make things simpler--but mainly to give a defensive boost to more experienced combatants.

Obviously this is very raw, and I'm not sure I'll ever do the calculations/testing to hammer it into shape. But I wanted to get these ideas down before I forget them, and anyone else who wants to play with them might get something out of them.