Like most d20 systems, Darkmatter d20 uses a roll of a 20-sided die to decide most actions.  For the most part, the rules presented in the Player's Handbook work just the same for Darkmatter d20.  Characters make their attack rolls using the same modifiers a Barbarian with his battle aze would.  However, a few major rules systems do need a little tweaking, to match the flavor and spirit of Darkmatter.

Hit Points and Wound Points
Hit points are no longer the end-all be-all of how much abuse your character can take.  Characters now not only have Hit Points, but also have Wound Points.  A character's Wound Points total is equal to their Constitution score, and can be raised by the Vitality feat.  Everytime a character takes a critical hit, the damage from the weapon that would normally be dealt to the character's Hit Points is instead dealt to their Wound Points.  A critical hit with a melee weapon does its regular damage multiplied by its normal critical hit modifier to the victim's Wound Points. This makes melee weapons, like knives, much more deadly. Once a character reaches 0 Wound Points, they are in a very serious catatonic state.  Unless they are bandaged within the next minute, they die.  Wound Points are healed at the rate of 1/day.  Hit Points follow the same rules as in the PHB, until they reach below 0. From that point on, the character' loses 1 Wound Point per round until bandaged.  Once a character's Wound Points is equal to or below the half-way mark (round fractions up), than they suffer a -1 Action Check penalty, to the same skills that are affected by armor, due to the strain of their injuries.  Once a character's Wound Points fall equal to or below their 1/4 point (round fractions up), than they suffer an additional, cumulative, Action Check penalty of -1.

Combat Improvement
The manner in which characters improve in combat has changed, also.  No longer do characters automatically improve in ranged or melee combat.  Instead, they have to spend skill points on the skills Firearms and Melee Weapons.  More information on this can be found in the Darkmatter Skills for d20 section.  However, GMs should be aware of this, as it makes lower-level characters much more capable in combat.  Crossbows use the Firearms skill to fire, as do all other primitive ranged weapons.

Feats, Quirks, and Increased Ability Scores
Feats and Increased Ability Scores are achieved at teh same rate as in the PHB.  Characters in any class gain feats at 1st, 3rd, 6th, etc., levels.  No classes gain extra feats, like the Fighter or Wizard do in D&D.  Instead characters may purchase extra feats by also taking Quirks.  More information on this can be found in the section labeled Darkmatter Feats for d20.  Note, also, that most characters are humans in d20, and therefore gain an extra feat at 1st level, and a number of extra skill points.

Combat
Combat is handled in Darkmatter just like in D&D.  However, the advent of modern weapons has changed a few aspects of the system.  Characters who are using guns with the Burst option of firing can fire three bullets for every one of their attacks.  They make one attack roll.  If that roll hits, the character hits with one bulllet.  Than subtract 2 from the roll.  If that still hits, than the second bullet hits.  Finally, subtract another 2 from that roll.  If that hits as well, than the character hits with all three bullets.  To determine the total damage, take the damage of one bullet (generally 2d whatever), than add 1d whatever for every bullet past the first that hits.

Characters with automatic weapons have a variety of special options they can take with their own weapons.  They can make an Aimed Burst for any single attack of theirs as well.  The burst releases 2d4 bullets.  Handle this just as an Aimed Burst from a Burst mode weapon, except subtract 3 for every bullet past the first from the attack roll.  Also, characters with automatic weapons may spray an area with bullets, of up to 90' width.  Everyone in this area must make a Reflex save (adding in bonuses for Cover, and their armor's AC adjustment) vs the shooters Firearms roll.  Anyone who fails their save suffers damage as if hit by a single bullet from that gun.

The rules above are just a basic guide.  Some topics may have been left out that need to be covered.  If you have any questions or comments please e-mail us.