Anyway, I was thinking about aimed hits and how unapproachable the rules I know are. At least it's something I have to look up all the time if it comes up in play. The real problem I see, is in the fact that damage is constant, but hit points are not (most of the time, there is the idea of damage by size). Ideally I'd need a rule that's easy to remember but allows for higher damage on higher levels and dismemberment even with big monsters.
How to kill a king with a sniper.
My initial approach was along those lines. Some random king is on a parade and a professional sniper wants to take him out. I don't like the rules for assassins, but they point out the problem pretty good by establishing another subsystem of rules to make it possible. If a king is level 9+, he has enough hitpoints to shrug off almost any "natural" damage going his direction other than a critical hit. But a critical hit is luck, it can't happen by design.
First part of the solution is to allow backstabbing for ranged attacks. I let thieves choose with character creation if they either specialize in dealing extra damage with ranged or with melee attacks, when possible. But it's still static and even with double damage and a lucky roll it's unlikely to snipe a high level enemy out of existance.
The second part is to find a key to estimate how much damage is needed to render a limb useless or even dismember it and it needs to be related to any number of hit points imaginable. Can't be too low, can't be too high.
Most of the time aimed hits are connected to fixed negative modifiers. Again, this is static and doesn't help other than making it more difficult to hit a location. Anyway, it's a start. The idea is to allow the players to choose how high the negative modifier is, before any positive bonuses apply and, if they hit the target, allow this negative modifier as extra damage. So if a player chooses to aim for a leg with a -10 on the attack roll, he gets +10 damage if he hits the target (in addition to hitting the leg).
This way the dealt damage is strongly connected to the ability of a given character to hit stuff (that is for the biggest part: level).
Now for the dismemberment.
Just working with what a basic monster description is giving a DM, this needs to factor in level/hd and AC. Let's assume a limb can take around 40% of the hitpoints a creature has, before it's rendered useless (one may try and give different limbs different values, like Hackmaster or Runequest do, but I'd like to keep it simple and chose a more conservative estimation to cover all...). Just add the AC to that and it's good to go.
Result: 3 x hd (or level) + ac-value (take it all, magic, dex, protection, whatever)
= damage needed to dismember or cripple
The king is dead, long live the sniper!
To kill a level 9 king, it needs 27 points of damage to the head (I'd allow a save vs. death with attacks aimed at the head) plus protection (plate armour would be +7 = 34). A high level thief taking at least -12 for the hit and backstabbing should be able to pull it off:
(1d6+12) x 2 for damage (again, with basic assumptions,
take extra damage from magic or whatever into account and it's more...)
On lower levels it won't have that much impact (other than flavour), because of the low hit points (but armour gives low level characters an edge). For higher levels, though, it stays flexible and easy to remember and calculate. You just need hd or level, the negative modifier the player chooses for the hit and the targets AC.