Never really satisfied with the original GURPS rules for handling defense in combat, I've decided to design some new ones, and here they are.
There is no more Passive Defense. Any current PD scores are ignored, using just the armor's DR -- the only exception to this are shields, i.e. anything actively used to defend.
There are still three defense maneuvers, and they are treated as such (according to the Maneuvers rules from GURPS Martial arts or Compendium I):
Block: This maneuver defaults to the Shield or Buckler skills, modified by the shield size (see below). Cannot be improved.
Dodge: Defaults to DX-4. It cannot be improved as a maneuver, but instead a new Physical/Easy skill of the same name can be learned. Note that some Martial Arts styles consider Dodge a very important skill, and it should be included in it as a primary or secondary skill; some examples are the western boxing and some European fencing styles.
Parry: This maneuver defaults to the skill with the weapon it is used with, with modifiers depending on the weapon type (see below). Cannot be improved.
When an attacker rolls a successful attack roll, the defender chooses one of his available defenses and roll: if he makes his roll by an amount equal or greater than the one the attacker succeeded by, he has successfully defended.
Example: The attacker has a skill of 14, and rolls a 11 -- success by three. The defender has to succeed by three or more to defend himself.
In case of an attack that can be defended against (usually Dodged), but was not aimed directly against the character, such as avalanches or explosions, the GM should apply an appropriate modifier to the character's defense roll.
Various modifiers can affect the Defense roll, depending on the Defense Maneuver used and other circumstances.
The primary factor affecting the Parry roll is the weapon used for it. Generally, it can be said that a small weapon is harder to parry with, while with very large weapons their size gets into way. We can then divide weapons in several groups, each of which have its own modifiers to Parry roll:
Although counted as a separate defense maneuver, Block is actually nothing but a Parry with a shield. Although it's usually used with shields, a character can use anything to block an attack -- from a beer mug to a trashcan lid.
A successful Block usually means that the attack was turned by the shield, which has taken most of the damage intended to the defender, per sidebar on p. B120. However, with these rules, damage to the shield is calculated on any successful Block, no matter how much it was succeeded by.
One of my pet peeves with many roleplaying games, including GURPS, is that the quality of an attack has no bearing on the damage it inflicts, instead relying on a random roll. It's very frustrating when you character rolls 5 points below his skill, and inflicts only a minimum damage to his opponent.
To mend this, I have introduced this simple rule: instead of rolling for damage, add one the success margin (the amount the attack was passed by) with the number of dice of damage the weapon inflicts. If there is any modification to the roll, apply it after the multiplication; also, treat any success margine above five as if it was five.
This modification gives the same damage range as the classic roll, only with less detail if there are more dice. For example, if the character attacks with a Winchester '73 rifle and succeeds by 4, the target receives 3 x 5 = 15 points of damage; if he succeeded by only. Or, if a ST11 character swings at someone with a greatsword, making his attack by 2, he will inflict (3 x 1) + 4 = 7 points of damage; if he made the roll exactly, the damage would be only (1 x 1) + 4 = 5 points.