I've changed the way shields work so they're more in-line with the way armor works. Rather than granting a defense bonus, they provide DR against attacks that strike them.
Why?
Imagine this: you're a Greek soldier standing with your comrades facing off against a Persian army. You're in a field with grass up past your eyes. A cloud of dust so thick it blocks the noonday sun fills the sky. You have only your helmet, your shield, and your spear to protect you from the enemy. The arrows rain down upon you. You don't see them. You can barely see the men at your side. You raise your shield to protect you... and the GM speaks: "You don't see the arrows. No defense roll."
Your shield might as well not exist. It doesn't exist, as far as GURPS cares.
Another scenario: you're a Viking warrior. Much like the hoplite, you're armed only with your helmet, your shield, and your spear. You fight in much the same way as the Greek before you and the Sumerian before him: in a mass of men standing shoulder to shoulder with a shield in one hand and a spear in the other. Your foe is much the same, though unlike the Greek fighting the Persians, you fight your neighbors from across the river. There are no ranged weapons on either side, only spears and shields and the rare man on horseback. You march forward and press your spear toward your enemy, holding your shield close. Your brother is on your left, your uncle on your right. Not far off, you can see your father. As the mass of men collide, your brother takes a spear to the face. He falls to the ground. All thoughts of your own safety disappear and you enter a violent rage. You make your strongest thrusts at any opening, striking at maximum distance. Your shield is still there, though, between your chest and your enemy's spear. Your enemy stabs at your torso, and... again the GM speaks, "Yeah, he hits you in the torso. Four injury to your vitals."
In GURPS, any time you can't defend, or fail to defend, your shield is nowhere to be found. The space it occupies is considered empty by the game. Where there should be a large piece of wood and leather and metal, there is nothing. The enemy never accidentally strikes your shield. The enemy never takes deliberate action--at a penalty to hit--to get around your shield. Because it simply isn't there.
The rules:
Shields now provide DR as cover rather than a bonus to defend. Shield DB is no longer a thing. You may Block with your shield as normal, but it provides no bonus to your Block roll. It also provides no bonus when dodging or parrying. In this way, Blocking is in essence identical to parrying. Perhaps it would be best to abandon the notion of Blocking as a separate concept altogether and only have dodging and parrying, with shield parries being identical to parries with any other weapon.
Shields are treated as cover all the time. Whenever you have a shield near your body, it's providing cover against both melee and ranged attacks.
First: apply the "thin slab" rule for determining shields' cover DR. Ignoring this rule results in obvious nonsense cover DR values for shields. The Roman Scutum in Low-Tech has a Cover DR of 10. More than all but the thickest steel armors! There's no way that's right. So we'll be using its actual DR, without adding any of its HP, which gives us a DR of 4. Much better. This is a good place to note that these rules are intended to work with the realistic damage values of The Deadly Spring, and some theoretical similar treatment for melee weapons, slings, and so on.
Next, these work just like normal cover. It's a -2 penalty to bypass cover. This number may need to be revisited, but it works well enough for now. Note that massed missile fire never does this.
I've attempted to formalize what areas are protect by shields in different circumstances, but this will obviously need GM judgment to adjudicate at times. That's a feature rather than a bug. If you want to cover your shins with your shield, then the GM will have to determine how much your knee is protected. This is normal for cover! All the cover rules work this way. It's no different from if you kick over a table and stand behind it. I've given some examples, but these certainly aren't intended to be all-inclusive.
Also, for the sake of completeness, I've added protection chances for when you are presenting a single side of your body toward your opponent. We don't exactly have rules for this now, but these values will be useful later on when we do. I'll do another post on that some other time.
Otherwise, these work just like armor. You have a location and the shield provides DR for that location. Some of these provide only a chance. For instance, if a shield gives a 4/6 chance to protect an area, roll a six-sided die. On a one, two, three, or four, the shield protects. On a five or six, you receive no protection.
One more thing. The dueling buckler is a special case. It provides no cover, but gives DR 4 to the hand. It can block as normal.
All shields provide cover only against attacks from the shield side and the front. If other areas are protected, such as when on horseback, or when the shield is carried on the back, GM judgment will be necessary, as I have no formal rules for those situations as of yet.
The short version: shields give DR, just like armor. It's a -2 penalty on your attack to bypass the DR. This replaces all the other stuff shields did. You can still Block, but shields give no bonus to any of your defense rolls.
Here are my examples. These are meant to demonstrate the concept. I make no claims of their exact accuracy.
Hoplon or Scutum - DR 4 Front/Weapon Front/Complete Skull - - Face - - Neck - - Chest - 3/6* - 6/6 Abdomen - 3/6* - 6/6 Groin - 6/6 - 6/6 Left Shoulder - 6/6 - 6/6 Left Upper Arm - 6/6 - 6/6 Left Elbow - 6/6 - 6/6 Left Forearm - 6/6 - 6/6 Left Hand - 6/6 - 6/6 Right Shoulder - - 6/6 Right Upper Arm - - 6/6 Right Elbow - - 6/6 Right Forearm - - 6/6 Right Hand - - 6/6 Thigh - 3/6* - 6/6 Left Knee - - Left Shin - - Left Foot - - Right Knee - - Right Shin - - Right Foot - - *Increase to 5/6 if presenting shield side. Kite Shield - DR 4 Front/Weapon Front/Complete Skull - - Face - - Neck - - Chest - 3/6* - 3/6* Abdomen - 3/6* - 3/6* Groin - 3/6 - 3/6* Left Shoulder - 6/6 - 6/6 Left Upper Arm - 6/6 - 6/6 Left Elbow - 6/6 - 6/6 Left Forearm - 6/6 - 6/6 Left Hand - 6/6 - 6/6 Right Shoulder - - Right Upper Arm - - Right Elbow - - Right Forearm - - Right Hand - - Thigh - 3/6* - 3/6* Left Knee - 3/6* - 3/6* Left Shin - 3/6* - 3/6* Left Foot - - Right Knee - - Right Shin - - Right Foot - - *Increase to 5/6 if presenting shield side. Heater Shield - DR 4 Front/Weapon Front/Complete Skull - - Face - - Neck - - Chest - 3/6* - 5/6 Abdomen - 3/6* - 5/6 Groin - 1/6** - 1/6** Left Shoulder - 6/6 - 6/6 Left Upper Arm - 6/6 - 6/6 Left Elbow - 6/6 - 6/6 Left Forearm - 6/6 - 6/6 Left Hand - 6/6 - 6/6 Right Shoulder - - 6/6 Right Upper Arm - - 6/6 Right Elbow - - 6/6 Right Forearm - - 6/6 Right Hand - - 6/6 Thigh - 1/6** - 6/6 Left Knee - - Left Shin - - Left Foot - - Right Knee - - Right Shin - - Right Foot - - *Increase to 5/6 if presenting shield side. **Increase to 3/6 if presenting shield side.