Toward an Adequate Set of Combat Rules

My sincere hope is that if I keep honing this, I'll manage to get it right one day. By getting it right I mean making it understandable to everyone playing, reflecting what we know about the laws of physics, and simple enough that I know the rules cold, don't need to look at any charts, and can do all the calculations in my head right off.

It's generally accepted that needs to be fast-paced. During combat, the DM is visualising what's happening in detail and considering a number of possible consequences to each action. He is sometimes rattling off answers to questions at the rate of several a minute to players who don't want to--and shouldn't have to--wait for him to look up the answer. He is often planning for what might happen after combat. He will frequently be drunk. Modifiers are lovely in that they help explain variables and contingencies of the sort that happen when people are trying to kill each other, but they should be kept to a minimum at least until the DM knows the system like the back of his proverbial hand. 

Amour Class (AC) = Defence

I know it's been explained and house ruled into meaningful things by much better DMs than I, some of whom have blogs that are much more worth reading, but the idea that wearing armour makes you more difficult to hit is patently absurd. I've tried to compensate by saying things like like 'The sword blow glances off your gauntlet' in order to convey what would be likely to happen on an unsuccessful attack roll, but to paraphrase Russell Glasser, in order to count as an explanation, it has to make things more understandable. And our players are sometimes drunk. What I propose, then, is that AC simply represent defence and that it improve in tandem with THAC0. A fifth-level fighter, for example, would have an AC of 5, making him quite hard to hit regardless of what armour he's wearing. This should be quite hard to attain, as no character in our campaign has yet survived past fifth level. We might reason that there are rather few of such level in the world. 

Hit Points = 1/2 CON

At the suggestion of our resident Simulacres player, HP are taken directly from a character's Constitution score, rounded up to the nearest full hit point. (We toyed with the prospect of making HP equivalent to CON, but that seemed too high in consideration of the amount of damage most weapons do.) Characters begin to suffer significantly at -1 HP, losing 10% of their ability scores at each lost point, are dead at -10. (It shouldn't be hard to figure out from whom I pilfered this rule.)

HP are determined at character creation, and they don't change. This reflects the reality that people are harder to hit as they gain more combat experience, but if they are hit, they suffer the same damage as everyone else.

Somebody suggested that this makes things difficult for mages, who have to wait three levels before their defence improves. In fact, the average rate of HP improvement by third level under the standard rules is 7.5 for the mage (1d4 average of 2.5, multiplied by 3) versus the fighter's 15 (1d10 average of 5, multiplied by 3), so our system clearly gives him the better deal. 

Bleeding to Death

If a character is reduced to -5 HP or more, he will lose an additional 1 HP every five minutes until his wound is bound to stop the haemorrhaging. When he's no longer losing blood, the DM rolls a d20 to determine the severity of infection, unless the wound is treated with an herbal poultice or the like (infections start at rolls of 2 below CON and become progressively worse, with a natural 20 being fairly severe gangrene). 

Armour Defence

In our most recent trial setting, the only real defences that exist are hide armour, chain mail, and shields. Hide armour absorbs 1 HP of damage and is then torn. Either chain mail or a shield will deflect 50% damage on its own, rounded up; both used together will deflect 75% damage, rounded down. (A hard enough blow against one's shield will still hurt, and the shock could even break a bone.)

Chain mail quality is divided into three broad categories at this point. On every successful hit, mail of standard construction is torn on a 1 in 1d6; of fine construction in 1d12; and mastercrafted in 1d20.

The standard metal helmet absorbs 50% damage only if the hit location die reads 'Head'. At any rate, full damage is taken on a hit to any unprotected location. 

Modified Attack Rolls

Also per our resident Simulacres player, character may expend extra effort trying to hit exposed body parts. Obviously, this only works with armour that has significant gaps, but the only real armour we're working with presently is a chain mail shirt, which leaves the lower legs, head, and neck. At a 2 point THAC0 penalty, combatants may aim for any of these he chooses. At a 5 point THAC0 penalty, a combatant may attempt to behead his enemy; on a successful roll, hit location and armour are both ignored, and full damage is automatically sustained. 

When we ended our last session due to time constraints, the party was hot on the trail of a perceived enemy; if they find what they were looking for this next time, it will offer an excellent opportunity to test the system.