Like all great westerns, the Rambo franchise is a black and white tale of good versus evil. The guns might be bigger, but the song remains the same.
It'd be a kind of First Blood prequel, set in the Old West, focusing on the adventures of John Rambo's cowboy ancestor whose name, totally coincidentally, is also John Rambo.
He also dresses a lot like Rambo, and has a massive modern machine gun, which he uses to shoot hundreds and hundreds of cowboys. Which is sort of cheating, really.