Law of the Gun is a set of free wargames rules by Richard Bradley for Wild West Skirmishes.
Wild West Shootout
From Gettysburg Gamers: a set of free wargames rules for Wild West shootouts.
A Town Called Malice Wild West Rules
A Town Called Malice is a set of free wargames rules for wild west gunfights. The rules set was used by the North London Wargames Group for their 2015 club convention game.
Building A Western Gaming Table
The Great Escape Games forums have a very nice photo tutorial on making a western mine gaming table.
Wild West Exodus Free Core Game Rules
Wild West Exodus is a skirmish game that mashes the 19th century American West with steampunk and horror themes. You can get the free starter rules here.