Combat ROUNDS are reduced to 5 seconds! This makes combat far more realistic and believable. With a typical human semi-automatic weapon a character can get off 5 shots in a round, or his usual melee attacks, though a combat knife is no where near as effective as a machine gun! ;)
  A human's movement rate is 12, for 12 metres walking speed a round, or when running, 3 times that, 36 metres. Which is pretty close to the true speeds of someone carrying combat equipment.

  Explosives and incendiary weapons are generally NOT targetted at individual creatures!! Rather the area they are STANDING in! To hit a typical area one metre across requires an attack versus Armour Class 5. Also explosives can be used to get creatures round corners because of the blast radius, or with some weapons, lobbed over obstructions and fired "blindly" (with a -8 hit penalty), the intention being that the destructive radius will hurt or kill the enemy. This is the great advantage explosive weapons have: even a near miss will do damage!
  This rule will apply to throwing grenades and high explosives, firing rocket launchers, autocannon, heavy cannons, and certain alien weapons.

  For each point on the dice that an attack missed, the projectile etc lands 1 metre away from the target. For example, an XCOM soldier throws a grenade at an Alien, the attack is versus AC5 as its an explosive weapon thrown at the creature's area, not the creature itself! (see below). His THAC0 is 12, he rolls a 19, which is a miss by 2 (12+5-17), so the grenade lands 2 metres away from the Alien. Being an explosive weapon the Alien is still going to get hurt by the blast and shrapnel! Using the Explosive rules above, damage is reduced by 1 dice, doing 4d6 hp, the result is 16 hp of damage. the Alien is injured instead of killed, but it IS hurt!

  The destructive power of explosives (and shrapnel associated with them) is reduced by distance. Whenever an explosion occurs, its damage is reduced by 1 dice per 2 metres of distance of the target.
  For example, a High Explosive demolition pack is thrown at an Alien, the thrower ducks behind a nice stout wall and runs like a bastard before the 10 second fuse goes off! The explosive lands short by 4 metres, normally the High Explosive does 11d6 hp, but since it's 4 metres away this is reduced by 2 dice, so it does 9d6 hp damage. The dice are rolled, 48 hp, and the Alien is still blown to bits! This is why folk use HE weapons: even a near miss can kill you!
    Direct hits: When some poor unlucky sod gets hit directly by an explosive weapon (which generally requires an attack against their AC with a -4 penalty and they must be relatively motionless as well), they tend to be blown to pieces! To reflect this, in such circumstances they never take less than 1/2 the maximum damage the weapon can inflict. In the case of the hurled High Explosives from above, they would take at least 33 hp! 1/2 of  93hp (11d6=92hp)
  DM's may give saving throw versus Breath Weapon where the victim has some chance of getting out of the way, such as a grenade with the fuse too long etc.

  Incendiary weapons are fired at an area, just like explosive weapons, and the "misses" rules also apply.  What's particularly bad about incendiary weapons is that they STICK and burn for several rounds, they use thermite (a metallic paste) or napalm (an oil based material with other chemicals). these chemicals CANNOT be extinguished by any normal method! Only specialist, and very rare fire fighting equipment can put them out. The only way to get them off is to scrape them off, but that inflicts damage anyway due to cutting and the flames! (i.e. DM should rule on damage). Few creatures CAN try to remove incendiaries from their skin as they are in agony.
  Also, they tend to start fires, save vs. Normal or Magical Fire depending on the amount of heat. For instance, an incendiary rocket would be versus s magical fire as it can do 6d6 hp in one round, but an incendiary grenade at 3d4 hp would be versus Normal Fire.
  FIRES: a typical fire, burning car, house starting to burn etc, does 6d6 hp per round to anyone inside it, or 1d6 hp to all in 5 metres. A major fire, such as a house well alight or the detonation of a fuel tank, would do 10d6 hp per round, and 2 d6 hp to all in 10 metres. Note there may also be smoke, cutting down vision, and making it hard to breath (-1 hp per round of stun damage). There are some catastrophic types of fire that can break out, such as when a petrol (gasoline) tanker, tank of natural gas, or numerous gas cylinders, blow up. This causes a huge explosion AND a fire! Typical damage will be 20d6 over a 30 metre diameter from the blast (then damage starts to reduce), and the fire could ignite things up to 100 metres away!
  

  Critical hits are an optional rule. Any attack roll of 18-20 on the dice, or a failed saving throw of 1-3 where appropriate, results in a critical. A critical multiplies the TOTAL damage inflicted by 2d4, plus organic creatures take 1 hp per round form bleeding etc until they are tended to. Tending to a wound requires one round and needs the services of someone with the Healing proficiency or the Medi-Kit device, if availaible.
  Note that critical hits can ALSO be applied to Aircraft attacks!!
 Backstabs are considered automatic criticals if they hit (x2d4 damage)

  Aircraft attacks are carried out just like regular combats, with the opposing craft, UFO versus XCOM Interceptor.
  XCOM craft have an Armour Class msotly based on the skill of their pilot's, and the manouverability of the craft. A vehicles AC=10 - (pilots level+Dexterity AC bonus+craft's Acceleration rating).
  Thus a craft with a 5th level pilot (-5) with 16 Dexterity (-2) in an Interceptor (-3) is 0. Scores can go into minuses.
  If an XCOM vehicle is shot out of the sky, the pilots can eject, they are allowed a save versus breath Weapn -4 penalty to get out before the vehicle gets vapourized! If an aircraft is shot down the pilots die...

   XCOM personnel are NOT suicidal, idiotic heroes; they are human beings who want to SURVIVE! And because of that, they have Moral values just like NPCs! They do NOT WANT TO DIE!
  XCOM soldiers have a Moral of 15, or equal to their Wisdom score, or their level, whichever is better. This assumes they are Elite troops, such as Delta Force, SAS etc, other types of troops (Marines etc) have lower Moral, generally 14.
  The standard bonuses and penalties for Moral in Combat apply as per The Dungeon Master's Guide, such as losing allies, suffering 50% casualties etc.
  When moral breaks, troops are affected with Panic for 5 rounds, or until an officer can get to them and speak to them, in which case they are allowed a save vs. Spells adjusted by Wisdom and improved by the Officer's Charisma Encounter Reaction bonus every round.

  When an XCOM soldier or Alien creature panics, i.e. fails a morale roll, it will do one of 3 things, roll d6: 
STUNNED: The victim is stricken with fear and can't do anything but cower and sahek.
FLEE: The victim drops all held items and runs like bastard in a random direction to get away from combat and danger!
BERSERK: The victim manically fires their weapon at random, trying  to preserve themselves! However, they can blow themselves up if armed with a rocket launcher and firing at the wall infront of them!! The DM should roll 1d4: 1) fires at own feet! 2) Fires very close 5d4 metres away; 3) fires at medium distance 10d6 metres away; 4) fires far away 20d10 metres away.
  Obviously, going berserk inside a building, where everything is close, could be very bad....
   When moral breaks, troops are affected with Panic for 5 rounds, or until an officer can get to them and speak to them, in which case they are allowed a save vs. Spells adjusted by Wisdom and improved by the Officer's Charisma Encounter Reaction bonus every round. 

  XCOM personnel in a base recover 3 hp per day, thanks to medical attention.

PLAYERS SHOULD ONLY READ THE PSIONIC SECTION WHEN THEY HAVE RESEARCHED THE APPROPRIATE TECHNOLOGY!

   Psionics in XCOM aren't like the usual psionics in AD&D. To begin with, humans CAN'T use psionics until they have researched the Psi-Amp device. Next they must build a Psi Lab in a base to train folk in how to use psionics.
  Psionic creatures can make a psionic attack once per day per point of Wisdom they have (Intelligence for Aliens). There are two kinds of psionic attack: Panic and Control.
  PANIC: The victim is allowed a saving throw versus Spells, adjusted by Wisdom bonus saves. If the creature or human fails, they panic. See Panic, below, for details on the results. Panic lasts for 5 minutes.
  CONTROL: this is much harder to achieve. The victim is allowed a saving throw versus Spells with their Wisdom bonus, plus a +4 bonus. If they fail, they are under the total control of their master, just like a Domination spell! The creature obeys its master to the best of its abilities, but when a suicidal order is given, they are allowed another saving throw roll to break the mental grasp! Control lasts until the "master" breaks it, or is killed or is more than 1 mile away. Just like a Domination spell, this means folk can be controlled for long periods, see the Charm Person spell for how often a re-roll for saves is allowed. Fortunately, only that SPECIFIC creature can control the victim.
  Note that Aliens can capture people and affect them with deep psionic conditioning... 
  XCOM PERSONNEL: After one month of training at a Psionic Laboratory, XCOM personnel with a Wisdom of 15 or more become multi-class (or dual class if they have 17 Wisdom), adding PSIONICIST to their current class. They use the experience, hit points (and attack and saving throw where appropriate of) of Thieves. For every 5 levels they gain, their enemy suffers a -1 saving throw penalty. Humans need Psi Amps to manifest their psionic powers.
  Optionally, the DM and players can allow, and create new psionic abilities, such as telekinesis. Also, some folk may be psionically weak, there being a   in 10 chance any human gets a -4 saving throw penalty against psionics,.perhaps the Aliens have been experimenting with previous generations to make us easier to control...?