Problem with that is it would be impossible to make into a game that would be even remotely popular. The style of warfare just doesn't work in games. And if you don't use that style of warfare, but let people run around in little deathmatch fights, it'd just devolve into people running around and stabbing each other or shooting at point blank range, rather than using their shoddy ranged weapons.
Having said that, I obviously forgot about the old west. It could be really awesome, if done properly. Shame that's so rare. Don't get me started on how much Red Dead Redemption sucked. The original Call of Juarez was pretty original and awesome, with the sequel being slightly less original, and definitely lacking in some areas, but still definitely a good game. The biggest problem as I see it would be that despite being a western game, it'd still play like a modern one.
Not that CoD wouldn't be fun in an old west setting. Killstreak? Covered wagon with gatling gun! Off-map cannons! Cavalry charge!