Critical Hits
Critical Hits work differently in CoD than in D&D.  The crit range remains, and to score a critical hit, the character must roll within this range.  However, for the critical hit to succeed, the character must also have also rolled at least 5 above the number he needed to hit his opponent.  This eliminates the need for a second attack roll to determine whether or not the attack was critical.  However, instead of dealing multiple times the normal amount of damage, a critical hit deals damage directly to an opponents Wound Points (see Damage).  Note that melee weapons still deal multiple amount of damage, and also do it directly to the character's Wound Points.

Defensive Maneuvers
With the lowered effectiveness of armor and loss of feats such as Expertise, it is far easier to be hit in CoD than in basic D&D.  This reflects the lethality of modern combat.  This is why it's important that characters realize the significance of cover, and begin to understand taking defensive positions.  The PHB gives a +2 bonus to AC vs ranged attacks for people who kneel, and a +4 bonus to AC vs ranged attacks for those who lie prone.  Take these positions to improve your Defense Score, and make your character harder to hit.  Lying down prone and getting back up are each partial actions.  Kneeling is a partial action, but getting up from a kneeling position is a free action.

Multiple Hits
The most damaging part of being shot isn't necessarily the bullet.  When one is shot, a wave of concussive force goes through their body, caused by the impact of the first bullet.  It shocks the body, and rips through and destroys neural pathways, capillaries, and a number of other important systems.  Because this wave is not easily doubled up, rapidly successive hits from a bullets do less damage than the first round that hit.  It is for that reason that, if any character is shot by a Burst or Spray attack, hitting them with multiple bullets, they do not take full damage for those subsequent rounds.  Instead, only the first round does full damage.  All others do partial damage of 1dx, with x being the same die of damage the weapon normally does.  This only applies to bullets from the same gun, from the same character, during the same attack.
Example- Al's character is hit by a burst of SMG fire, by 2 out of the 3 rounds fired.  The SMG normally does 2d8 damage.  Since Al was hit with two rounds, he takes (2d8) regular damage + (1d8) partial damage, or 3d8 total damage.  Next, Al's opponent gets a second attack (as he's a high-level villain), and takes an Aimed Shot at Al.  Once again Al is hit.  However, this time he takes the full 2d8 damage again, as this was a different attack than the first burst which hit Al.

New Combat Maneuvers
Modern weaponry provides characters with a whole new myriad of combat moves.  D20 rules for these are presented below:
Aimed Shot- a single attack, any character with a gun in single-shot mode may make a single shot, without taking into account their recoil penalty, at their opponent.  This is a partial action.
Aimed Burst- a single attack only capable of being attempted by characters using weapons with a Burst setting, this option allows a character to essentially make a Burst attack (see below), but without having to reduce their attack roll by the recoil penalty for their first shot.  This is a partial action.
Automatic Burst- a single attack that can be tried only with automatic weapons, the attacker unleashes a burst of fire at a single opponent.  That opponent must make a Reflex save as if they are under attack from a Spray.  An automatic burst drains 5 rounds from the weapon's magazine, with fractions rounded up.  Anyone within the line of fire of the automatic burst must also make a save.
Burst- a single attack that can be tried with semi-automatic weapons or revolvers, this option allows a character to fire three rounds in rapid succession at a single target.  The character makes a single attack roll, subtracting their recoil penalty once to see if their first round hits, twice to see if the second round hits, and a third time to see if the third round hits.  The rounds do damage as Multiple Hits.
Cover- any character using a ranged weapon may spend his full-round action covering a particular direction.  He may than make a ranged attack of opportunity against anything which moves into that area between his current turn and his next turn.  This can be an Aimed Shot, Aimed Burst, Automatic Burst, Burst, CriticalHe may only make his normal number of Attacks of Opportunity per turn.  This has the same affect on Initiative as the Delay maneuver.
Critical Shot- a character with a ranged weapon using single-shot or burst mode may make an attempt at a Critical Shot.  Anyone making a critical shot must make an attack roll vs their opponent with a -10 to hit.  However, if the target's armor does not cover the part of the body being aimed at (generally hands, feet, or head), than that armor does not protect the target.  The shot is treated as if an Aimed Shot or an Aimed Burst.  If the shot hits successfuly, than it is treated as a Critical Hit.
Spray- a full-round attack, characters with automatic or fully automatic weapons may spray an area of 90 degrees or any measure smaller than that, draining 12 rounds held in the weapon.  All characters in that area must make a Reflex save vs DC 15 for automatic pistols, DC 20 for sub-machine guns and automatic assault rifles, DC 25 for light and heavy machine guns, and DC 30 for mini-guns.  Targets get regular bonuses for Reflex saves from cover and armor (see Arms & Armor), as well as bonuses for moving or changes in stance.  Characters failing their saves take damage as if hit by one round, plus one round for every 5 points by which they failed their save.  To succeed at a dodge, they must dive to the ground, unless they are behind cover, and therefore start the next round lying prone.  They may choose not to dodge, and take damage as if hit with three bullets.

Variant Rule: Combat Haze
Few things are as distracting as combat.  With guns blazing and grenades exploding everywhere, it is hard for a person to get any work done.  This rule allows GMs to bring this aspect of the real world into their game.  If they are within 20' of someone who's last action was to fire a gun or engage in melee with anyone, or if they were the target of an attack last turn, or if they were within 30' of an explosion the round before, make them make a Willpower check (DC 15).  If they fail, they suffer a -4 to any action not directly related to combat, such as using a computer or picking a lock.  This only lasts for a round, though they may have to make another check their next turn.  The Cool-Headed feat lessens this penalty.

Variant Rule: Point Blank Attacks
No matter how great a soldier, a bullet to the head is almost certain to kill them.  This optional rule allows a GM to let his character's take advantage of this basic fact.  If they're within 5' of an opponent with a ranged weapon who is either helpless, grappled, or completely unaware of the character's presence, he may make an Aimed Shot, Aimed Burst, Burst, or Spray that acts as a Coup de Grace, without saves or other defensive rolls.  The DC for the Fortitude save is equal to the damage dealt for firearms.

Variant Rule: Stuns
In modern combat, getting hit has some serious consequences.  If a character ever takes damage greater than their current total of Wound points, they must make a Fortitude check (DC 15).  A failure indicates the character is Stunned for the next round (DMG p.85), while a success indicates they are only Staggered for a round.  Every time a character takes Wound point damage, they must make the same check.  Success indicates they are Stunned for a round, while a failure indicates they are Stunned for a round, and knocked prone.

The rules above are just basic combat rules.  If you have a question, or have found something which we left out, please alert us to it by e-mailing us.