These are house rules used for a GURPS Old West campaign based on the Elimio Estevez tribute to spagetti westerns called "Dollar for the Dead".
Characters built on 150 points + up to 25 in disads (including the required 15pt Enemy: Regulators) + up to 3 quirks. GURPS Basic and Compendium I allowed. Recommended stats:
Even though this will be a fairly cinematic, combat-emphasized game, you will need to use other skills besides the recommended combat skills above, remember those are just if you want a decent gunslinger character.
We will use Advanced Combat (except movement may be a little abstract: it will take you 2 turns to run from the wagon to the doorway... and ranges will be "about 30 yards or so" which will speed things up and is realistic anyway.)
All characters will have a Nerve attribute, equal to 10 +- (Strong/Weak Will) + Fright Check modifiers (Cool +1, Composed +2, Collected +3, Fearlessness +1 per level)
In any imminent gunfight situation, a character must make a Nerve Roll, failure means the character has become "rattled" and affects the ability of the character to function. This roll is of course highly modified by situation. A critical failure means a roll on the Fright Check failure chart. These affects are as per the "Gun Shy" rules in GURPS High Tech. (basically, a negative modifier equal to the amount failed for just about everything for the duration of the hostilities.)
In any confrontational situation, with the threat of gunfire, involving 2 or more potential gunfighters, (i.e. a "showdown" basically both fighters are attempting to rattle the other and is a very phsychological battle in itself.) This is handled by a Contest of Nerve Roll. This is modified by reputation. If someone has a reputation as a deadly quick draw or having nerves of steel or whatever and the other combatant is aware of this reputation, he is affected.
Alcohol increases nerve attribute.
Remember that the Will score used for Nerve is based on 10, not IQ. Nerve must be increased through strong will or related advantages. (Sometimes stupid people are harder to rattle sort of thing.)