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I'd heartily recommend A Fistful of Lead.Meets all the requirements you've listed and comes with scenarios in the book and a second separate scenario book is available.The mechanics are easily picked up in a first game and there is a lot of characterisation through traits.Our group has been using them for a couple of years now, initially for Old West but of late we've done VSF, Gothic Horror and Cartel Wars. We've also played Napoleonic, F&IW and Smugglers with the Horse and Musket supplement.Give them a try!
There's a whole Rules for Old West thread around here somewhere, but I also heartily endorse Fistful of Lead. I also wrote them.
I have to agree -"Fistful of Lead" and "Dead Man's Hand" both fit the bill - DMH possibly has a few more modifiers than FFoL. The Rules With No Name are also good. And as the good Baron says, there is a rules sticky at the top of the board page, although it doesn't go into to much detail about actual game mechanics.RMZ
We really like Dead Man's Hand. A real cinematic feel, with cards used both for activation, and played from hand to cause stumbles, jams, allow characters to survive and so on.
You may enjoy "6 gun sound" by "two hours wargame".
+1 for this. A new version has just been released. See here - http://www.twohourwargames.com/sixgunsodeel.html