All I know is what I have browsed online and in the rulebook. So here's the benefit of my ignorance.
.357 Magnum is common in CAS. In fact, if you're trying to game the system, you'll shoot .38 Specials in a .357, and probably lightly loaded ones at that. (I presume you know that the .357 case is just a .38 Special case made a little longer, so that you couldn't accidentally fit a .357 into a .38 Special revolver. But there's nothing wrong with firing .38 Specials in a .357. There are a VERY FEW cases where it's safe to fire "some other" cartridge in a given firearm. .38 Specials in a .357 is one such case.)
You can't fire full power .357 cartridges in CAS because they're going to exceed the muzzle velocity limits allowed for those steel targets they use. If I remember correctly you need unjacketed lead bullets at 1000 fps or under. But they do make cowboy action .357 cartridges, and they certainly make .38 Special ones.
CAS has several classes for revolver shooters. A Ruger with adjustable sights would go in the modern category. From what I read, you can also fire one fixed-sight revolver and one adjustable-sight revolver in the modern category, if you happen to have one revolver of each type.
I suppose I should note that the matches require two revolvers, a rifle, and a shotgun. Maybe the main reason they don't have a one-gun class for beginners is that it would remove the excuse to go out and buy more guns.
Ruger single actions are popular in CAS, as anywhere single action revolvers are used, because they're a good quality gun and very reliable. The New Model Blackhawks, Vaqueros, and Single Sixes are even safe (if they're working properly) to carry with all six chambers loaded. This is not true of Blackhawks or Single Sixes built before the mid-70s, and it's not true of original Colts and their clones at all. These you carry with the hammer down on an empty chamber. Because CAS is designed around the Colt, CAS events also have you load five and have the hammer down on an empty chamber, even if you wouldn't have to do so for safety reasons.
