This is an expansion of the Trigger-happy note in the ‘New Disadvantages’ section of the ‘Houserule’ page of the GURPS Wiki.
Trigger-happy (-10 points in high-tech campaigns, -5 where firearms are not commonly used)
Whenever surprised, you will react with the maximum aggression at your disposal. This is an instinctive reaction, and happens regardless of how important it is to keep quiet to hide or achieve surprise, and you will not pause to identify the target exactly (i.e.: you will shoot your friends if they surprise you, or slash them with that knife under your pillow if they wake you).
If you have a firearm you will shoot, fast-drawing it if necessary. If you need to draw the weapon and don’t make a Fast-Draw roll, it will count as an All-Out Attack, drawing the weapon as the first half of the action instead of ‘move and attack’. However, this rules out the other options listed under All-Out Attack – instead of two attacks or a bonus to the attack, you will get the chance to draw and shoot. (This is the only situation in which Fast Draw can be used as part of All-Out Attack.) If you don’t have to draw the firearm (or if you make your Fast-Draw roll), you can shoot and dive for cover if you like. But you will shoot first.
You will not take time to Aim, accepting the snap-shot penalty in the interest of ‘getting off the first shot’. You also won’t change the setting on automatic weapons: if it’s on Auto, you will fire a burst rather than a single shot. (You will instinctively flick the safety-catch off any weapon if it’s on, and automatically push it through to Auto if it can do that!)
If have a melee weapon in hand and are close enough to the person who surprised you, you will do an All-Out Attack, striking as hard as you can [choosing the +4 damage option]. You don’t get the option of not doing All-Out Attack or taking the more skilled alternatives listed in its description – this is an instinctive maximum-aggression response. If you have no firearm, but can draw a melee weapon and reach the target, you will draw and attack as with a firearm above. Given the choice, you’ll draw a firearm rather than a melee weapon (remember: ‘maximum aggression’).
The Trigger-Happy attack is a special case if you also have Combat Reflexes. You will make your IQ roll to see if you are surprised as usual. If you would be surprised on the basic roll, you make the snap attack before your combat reflexes cut in. Then you consider whether the +6 for Combat Reflexes takes the roll above your IQ – if it does, you will be able to recover from the momentary surprise quickly and act normally thereafter. In other words, Combat Reflexes doesn’t neutralise the disadvantage of being Trigger-Happy.
Example: “So, to sum up. The target is a covert manufacturing facility in the north of Finland. We have evidence of a non-human workforce there. Although they are using indigenous animals to draw their vehicles as cover, we know that they have anti-grav transportation with an astonishing speed capability.
“The mission is to penetrate, investigate and neutralise this alien base.
“But there is one remote possibility that Research Div haven’t been able to entirely eliminate. So keep your hands well away from the trigger till you have definitely identified an immediate danger. Whatever you do, don’t shoot the fat jolly bearded man in the red suit!”