Correct me if I'm wrong but isn't aiming specifically for a body part considered a "Called Shot?"
Here's linkage to the specific feat/rules on that for Homebrew:
The details here give very specific easily applied consequences of a called shot--and a lot of people house rule called shots in, not as a feat but as a system like what your players are asking for... "Special: A DM may make called shot a variant rule instead of a feat; if this is the case, any character with at least three class levels in a class granting full attack bonus may use this feat."
In any case, a called shot is more DIFFICULT than a standard attack. What this means is that anytime a player asks for a called shot, THEY are at a negative to hit. Let's take the hand called shot, which according to the chart would likely be an arm called shot
Arm (−2 penalty): You hit one specific arm, forcing the struck creature to make a reflex save or drop whatever they were holding in that hand. If that creature had a two-handed weapon, it makes its save at a +4 bonus.
As you can see, when the player rolls to try to do this, you take -2 off their dice roll. They roll a 12, it's really a 10...
But if they are successful, they get the benefit of the CHANCE of the opponent dropping their weapon.
As a DM of 3.5 though, I am perfectly fine with the allowing the penalty and having the consequences. Here's the other ones other than the arm--
- Eyes (−6 penalty to hit): You hit your opponent right in the eyes, blinding it for 1 round unless it succeeds on a Fortitude save. Even if the save is successful, the struck creature is dazzled for 1 hour afterward.
- Face (−4 penalty to hit): You hit your opponent right on the face, dealing an extra 6 points of damage. It is also a deeply satisfying experience, granting you a +2 morale bonus to hit and and damage against this creature until the end of the next round.
- Teeth (−4 penalty to hit): You hit your targets right in the teeth; the attack is very painful, causing the target to be staggered for 1 round unless it succeeds on a fortitude save. Additionally, your attack knocks a few teeth out, disabling the bite attack of creatures who failed their save for 1d4 hours or until they receive magical healing (including fast healing and regeneration).
- Throat (−4 penalty to hit): You hit right on the throat, forcing the struck creature to make a reflex save or become silenced and unable to breathe for 2 rounds. If you wield a vorpal weapon, its decapitating effect activates on a 19-20 when you make a called shot to the throat.
- Torso (−2 penalty to hit): You hit sensitive tissues and bones on the torso, forcing the struck creature to make a fortitude save or be sickened for 2 rounds.
- Groin (−4 penalty to hit): You hit a very sensitive spot, forcing the struck creature to make a fortitude save or be nauseated for 1 round and a will save or be shaken for 3 rounds.
The rule here says:
The DC of these maneuvers is 10 + ½ base attack bonus + the user's strength or dexterity modifier (whichever is higher) and the save (and penalties) are made at the end of the current turn
But when I run, I simply use the target's AC as it is as the difficulty, with the negative penalties to hit depending on the body part. It's just simpler than trying to calculate all that other stuff.
And guess what! I let the NPCs do called shots as well--they take the penalty, and if they get the roll, there's consequences. The linkage I provided will keep things balanced for you if you decide to house rule it. You can do it as a house rule feat or keep it to the general house rule. It's really, really common as a house rule.
I personally enjoy the general houserule, and find that most of the time players don't love the negatives imposed to hit, so they don't end up doing called shots most of the time.
My advice: Tell them that you found the Called Shots Feat Chart, and that you'll allow them to continue to do called shots, BUT with the penalty for taking them, AND your NPCS will be allowed the same sort of effects. Because if the NPCs can do it too, they should/will be grateful for the limitations and the chances to do a save against the effects. If they aren't grateful go ahead and point out that they should be....
I will, if they roll ridiculously well on a called, sometimes allow something zany for the heroes, such as allowing them to partially cut off a hand, or take out and eye or something, just for flair, and then I won't bother with the chance to save...realistically though, they aren't going to be able to do that most of the time, and the effect is essentially what's on the chart--eyes, they have the blinded condition for a limited amount of time, and so on. I would not allow them to do a called shot where they say "want to cut off his hand" Let's just assume that they're aiming for his arm/hand and always WANT to do the max damage, but cutting off a hand is FAR MORE than an ordinary success.
Now, there are some issues--players might argue that the area they are trying to hit might not have armor and therefore armor should be ignored. DO NOT LISTEN TO THAT. The AC is the AC. See this article on how it unbalances the game.
Because if you allow something like that, even if it's smart on the character's part, two things can happen--1) it's easy mode and the the villains are smart enough to do the same 2) it seriously undermines the difficulty levels set out in the books.
Set the rules as a called shot being more difficult and stick to it. I don't know if this is what your players are trying to do (argue that if they go for unarmored bits like hands that armor does not apply) but if they are--this would completely undermine the entire system. To allow it to be played that way would mean a complete re-vamp of the 3.5 system. And then it's not D&D anymore... AC is one of the determinators of the XP award they get for kills and the difficulty level.
Sometimes a called shot is actually impossible--if the person has full head armor with a tiny slit for instance, I will let the player know that they can see that aiming for the eyes won't result in anything.