The American West genre of roleplaying games is a niche inside a niche. Boot Hill (1975-84), by Brian Blume & Gary Gygax of TSR had just enough of a gamer following to see further editions and adventure modules… for a time.
In 1981, Fantasy Games Unlimited released the Wild West RPG. It is definitely not a copy of Boot Hill, and in some aspects WW is arguably superior to its better known rival.
The rules are complex, using formulas and an “RPC Probablility Chart”. For those interested, the Wild West rules are posted forfree at a site even more ancient-looking than my own RPG reference site. Somebody went to a great deal of effort to type all the text in the rules booklet into the site. Very old school. I love it.