From the back cover:
Sidwinder: Recoiled is the ENnie Award winning d20 game of the Wild West By Dog House Rules. Capture the cinematic action and adventure of your favorite western movies, novels, and TV shows, using this complete set of rules. Create courageous gunslingers, low-down rustlers, or high stakes gamblers and test your mettle out on the frontier.
Sidewinder: Recoiled takes the d20 Modern rules and customized them for Wild West action. This rulebook contains everything you need for active gameplay.
Sidewinder: Recoiled is the definitive d20 treatment of the Wild West. Beware of fool's gold! So saddle up, strap on your hog-leg and pull your hat down low. You're fixin' to ride into the REAL Wild West of Sidewinder: Recoiled!
Requires the use of the d20 Modern Roleplaying Game.