Wounds & Damage
     Damage and wounds work differently in Call of Duty than in most other d20 games.  While most games take place in fantastic places, with strange beasts and terrific hero, Call of Duty is intended to be more realistic, with heroic, but not unbelievable, characters.  This is why Call of Duty uses two different systems to keep track of damage, Wound Points and Vitality Points.  Wound Point represent that core vulnerability and mortality that every person possesses.  Vitality Points, meanwhile, are a measure of a person's durability, energy, and willpower.  While Vitality Points increase at every level just like Hit Points, Wound Points remain the same, only occasionally being raised for Ability Adjustments and Feats.  Vitality Points are generally taken away every time a person is damaged by a normal hit.  While a punch, kick, or knive slash does only a little Vitality Point damage, guns can deal huge hits to a character's Vitality.  When a strike or shot deals a Critical Hit, that's when it does Wound Damage.  Wound Damage is that constant amount of damage that every human being can take.  It has far less variation through levels, and serves as a balance between all of the different character of the game, player and NPC alike.
     Every character starts with vitality points equal to his Constitution, plus a single roll of his Hit Die, plus his Constitution modifier.  Every level he gets a roll of his hit die, plus his Constitution modifier, and adds these to his Vitality Points.  Vitality points are removed by unarmed strikes, melee weapon hits, ranged weapon hits, falling damage, explosive damage, bleeding (see the Variant Rule below), and even fatigue (see the other Variant Rule).  Unless a weapon gets a critical hit, it does damage directly to the target's Vitality Points. 
     If a hit does get a Critical Hit, than it does damage to the target's Wound Points.  Every character has a number of Wound Points equal to his Constitution score.  Every point of Wound damage a character takes also inflicts a point of Vitality damage.  Wound points represent really serious injuries, that are dangerous to the character's personal health.  Not only can lost Wound Points slowly kill you, they also hinder your ability to do other things.
     When a character reaches 0 Vitality Points, he is considered mortally wounded.  He falls down prone and can do nothing but crawl 5 feet and talk until brought up above 0 Vitality Points.  Every round, he loses 1 Wound Point, until he is bandaged with a First Aid check of DC 15.  Once a character reaches 0 Wound Points, he lapses into unconsciousness.  Every round after that, he loses another Wound Point, until he is stabilized with a First Aid check of DC 20.  At -10 Wound Points, the character is officially dead.  If stabilized, he is in a coma until, at the regular rate, he is brought back to consciousness.
     Vitality Points are healed at a rate of 2 per day.  A First Aid check on the character of DC 15 every day allows them to heal an extra 2 Vitality Points.  Only one attempt may be attempted per "doctor" per person per day.  Wound Points are healed at a rate of one point per day, but only if the injured character makes a Fortitude save DC 20 every day.  He may substitute a doctor's First Aid skill, if under medical care.  Otherwise, he remains in an unconscious coma until he heals the proper number of points to bring him back above 0.
     When a character has a number of Vitality Points equal to or less than his normal, total amount of Wound Points, he is injured to the point of not being able to perform properly.  He may only take a single, partial action every turn when in this state, and may not move any faster than a walk.  He also suffers a -4 penalty to Strength, Dexterity, and Constitution checks.

Variant Rule: Fatigue
     Fatigue and tiredness can be a deadly thing in real-world combat.  Tired soldiers can't fight as well, or as long as fully rested soldiers.  In the game, Fatigue takes the form of a loss of Vitality Points.  For every hour of movement, a character suffers 1d4 Fatigue damage unless they can make a Fortitude check vs DC 15.  The Durable personal feat applies to this check.  If they are moving at a rate faster than their normal rate (such as at x2 or x5 movement), multiply this Fatigue damage by the same amount.  Fatigue damage is also suffered for remaining awake too long without sleep.  For every two hours past 12 hours that a character stays awake without sleep, they have to make automatically suffer 1d4 Fatigue damage.
     Fatigue damage should be kept separate from regular Vitality damage.  Fatigue damage can never kill a character; if it brings the character below 0 Vitality points, than the character does not lose Wound Points as normal.  They merely remain unconscious until the Fatigue damage heals normally.  Fatigue damage heals at a rate of 1 per hour of rest, 4 per hour of sleep.

Variant Rule: Bleeding
     In the real world, injuries that do major damage bleed, and bleed alot.  This normally isn't handled in an RPG, though at the GM's discretion, it can be.  If the GM desires, keep track of each individual hit done against each character that resulted in a loss of Wound Points.  For every one of these wounds, every turn, subtract a single Vitality Point.  So a character who has received three Critical hits loses three vitality points each turn.  A First Aid check DC 15 for each wound allows it to be bandaged, and stops the bleeding.

Variant Rule: Deadly Bleeding
     Some GMs may choose to make Bleeding even more deadly than in the rule above.  This is even more demonstrative of the real world, where bleeding, more than anything else, causes damage and death to people who are injured.  If the GM chooses, the character may suffer 1 point of Vitality Damage for every point of Wound Damage they are missing every round, until they are bandaged.