How much damage does a hit point represent?
For some time now, the official standard in D&D and Pathfinder has been that starting characters get the maximum hit points at 1st level, based on class. So typically 6, 8, or 10 - maybe 12 for certain fighting types or 4 for certain magicians, depending on the ruleset - plus a bonus for a high Constitution, which is pretty common.
You gain about half that amount, rounded up, plus the Con bonus again, each level. So it would be fairly normal to see an adventuring party with hp totals of 7, 10, 13 at 1st level; 12, 17, 22 at 2nd; and 17, 24, 31 at third.
I would also say it's standard that these hit points are treated as representing real, physical damage. In most games I've run, played in, or observed at an FLGS, the Game Master says something like "you hit!" and "you miss!" to describe combat results. They describe damage as being "just a scratch" or "a really bad wound". Characters recover lost hit points with the help of healing potions or healing spells, because those lost hit points represent bodily injuries that need to heal before the hit points can be restored.
(Let me digress for a moment to acknowledge that there are people who'll talk until they're blue in the face and you're blue in your soul about how well actually Gary never intended hit points to indicate anything so concrete as bodily health, etc, they have always and everywhere represented an abstracted reserve of luck, martial skill, fighting spirit, elan vitae, and character morale that gets depleted during combat, etc etc, anyone who says differently is playing the game wrong and ruining the hobby with their scurrilous misinterpretation of the founders' intentions, etc etc etc. I'm actually sympathetic to the argument that we could describe combat differently, but I think I'm on solid footing about how the game is usually played, and I don't find it useful to pretend that one's own preferred playstyle has some deep rooting in custom and tradition just to facilitate a rhetorical appeal to faux-historical authority.)
Most weapons use d6, d8, and d10 dice to deal damage, plus of course a likely bonus for high Strength. That means that depending on the match-up of character and monster and weapon, most starting characters can sustain maybe 2-3 hits before they run out of hit points, and can probably endure another 1-2 hits each time they level up.
So returning to my original question, how much damage does one hit point represent?
The Alexandrian argues that the answer is on a sliding scale. Suppose getting hit with a short sword deals 4 damage - how much bodily injury that 4 damage represents depends on whose body it is. For a 1st level thief with 7 hp, that 4 damage is over half their total. That's a pretty grievous injury. Another hit like that and they'll either be dying or just plain dead. For a 3rd level fighter with 31 hp, it's not so bad really. They could get hit 6-7 more times like that before it would kill them. The severity of the injury isn't determined so much by the number of hit points as it is by the proportion of the total.
DM David suggests that the abstraction of hit points - the fact that they don't easily map to any particular amount of bodily injury - is the reason for their enduring appeal. He observes that virtually every game that sets out to "fix" D&D's combat settles on some kind of rule to make hit points and weapons damage more "realistic", and that despite these many "fixes", D&D's decidedly un-realistic combat remains more popular. He argues that this is because it's more fun. Players like the positive feedback of actually hitting their opponents, and they like not dying instantly the first time an attack hits them. It's more fun to narrate combat as a trading of blows than as a series of dodges, blocks, whiffs, and misses.
When I first learned about D&D, before I started playing, I thought that 100 hit points per character sounded about right. I suppose I must have been thinking of hp as percents. If you'd asked me how long a fight should last back then, I probably would have wanted something like the duration of combat you get in a round of Street Fighter or Mortal Kombat. For "boss fights" anyway, although I don't know if the rhythm of lesser battles against weaker monsters leading up to the climactic crescendo of a "final boss" would have occurred to me then.
But some people want something different. They want combat to be short, decisive, and deadly. Or they want combat to be a "fail state", a mistake that you instantly regret making. Or they want it to be more "realistic". Or they want it to be more concrete and less abstract. At the risk of repeating the mistakes of the past, reinventing the square wheel as it were, I have a proposal for how to do that.
Hit Points as Bullet Wounds
- One hit point represents one bullet wound.
- The number of hit point a character possesses represents the number of times they can get shot before dying.
- The amount of damage an attack deals represents the number of bullets that hit the target.
- Attack rolls truly are "to-hit" rolls. If you roll a miss, that means the bullet misses its target.
- Your "armor class" represents your ability to dodge or evade. It's affected by your agility and by ducking behind cover, which causes the bullets to miss you.
- Body armor allows bullets to hit you, and so doesn't affect your "armor class" at all. Instead it provides something like a saving throw after combat to see if it successfully prevent the bullets from damaging you as much as they normally would.
- Hit point totals are low and will remain low. A non-combat character can survive being shot maybe 1-4 times, a combat-oriented character can survive maybe 1-6. Hit point totals probably don't increase as you gain levels, or only very slightly.
- If a character with 1 hit point gets shot, they start dying. If a character with 0 hit points gets shot, they're instantly dead. Most civilian NPCs have 1 hp. Having 0 hit points represents an state of illness or frailty.
- Dying will turn into dead unless you go to a hospital or other surgeon. Any character who gets shot will die from their injuries unless they take the time needed to apply competent first aid to their wounds.
- Any bullet wound that isn't treated in a hospital or equivalent will result in the permanent loss of 1 hp. Any bullet wound that is treated still requires something like a saving throw after treatment to recover, otherwise it's lost permanently anyway.
- Recovery times are long. Expect to spend something like 1 hour per bullet wound on first aid and something like 2 weeks per wound recovering afterward. And those might still be "unrealistically" abbreviated. Translate into your game's relevant "turn" and "downtime" categories as necessary.
- Most unarmed combat deals 0 damage. You can wrestle someone to restrain them, get in a fistfight in lieu of negotiation, maybe even knock someone unconscious, but your bare hands aren't likely to kill anyone, except under extraordinary circumstances. A critical hit might kill, even by accident, and so might beating a helpless person.
- Knives deal maybe 1 damage. You get something like a saving throw. If you succeed, you still need first aid, and will suffer the consequences without it, but otherwise you take 0 damage. If you fail your save, you need a hospital, and take 1 damage. Knives can kill, but not as easily as a gun.
- Most bullets deal 1 damage, but special guns might have special characteristics. A very weak gun, perhaps one that's very quiet or easy to conceal, deals maybe 1 damage, like a knife. A very powerful gun, or one that's firing very dangerous ammo, will definitely deal 1 point of damage and will maybe deal 2. Again, you need something like a saving throw. Armor piercing bullets don't allow you to make a save to prevent their damage, but let's say they only deal 1 damage if you're wearing a vest. Special guns and special bullets are very expensive.