I think SASS has their rule book on line.
Just going by memory, I think a Cowboy rifle is a lever or pump action of a pre-1899 design, with exposed hammer and tubular magazine. It shoots a pistol cartridge of at least .32 caliber, and may have a tang sight. It does not have a receiver sight, scope, box magazine, or concealed hammer. In general, if it would appear to be an anachronism in an episode of Gunsmoke, it would not be allowed.
I think there are some classes, like Plainsman, in which the competitor uses a single-shot rifle like a .45-70.
If you want to play, why don't you visit a match and talk to the players to see what they're shooting (and wearing, costuming is important to CAS).
