Were there any side-by-side buffalo guns back in the American Old West? Were there any side-by-side guns back then used to actually kill American buffalo?
No particular models that I know of.
Without a doubt, though, somebody along the way would have used a double barrel shotgun with a round ball on a buffalo. If noting else, a native American who managed to acquire shotgun would be able to use it just as he would a bow and arrow on horseback.