Pay attention - this is a short read, but confusing!
Some categories require certain revolvers, rifles or shotguns, some require fixed sights on your revolvers, some have caliber restrictions, such as .40 cal or larger, some require a gunfighter style holster rig (one on each side) rather than a crossdraw style. Classic Cowboy requires a double rig and holsters that position at least part of the revolver's grip above the belt, while B Western requires a Buscadero style rig that carries the revolvers completely below the top of the belt line.
A 12g or 20g SxS hammer shotgun is legal in all main match categories, as is an 1887 lever action shotgun.
Fixed sight cartridge revolvers of .40 cal or larger are legal in all categories.
Approved percussion revolvers of .36 cal or larger are legal in all categories.
There are NO smokeless powder categories... meaning there are no categories which REQUIRE smokeless powder. Black powder is legal in ALL categories.
There are NO categories which require adjustable sights on your revolvers, but there are some which require fixed sights. The old Modern category, which has gone by the wayside, DID require adjustable sights on at least one of the revolvers.
If you shoot percussion revolvers, add a '73 of .40 cal or larger, and you're good for all categories except B Western. Frontiersman requires percussion revolvers and B Western requires a rifle design of 1880 or later, so cartridge revolvers would exclude you from Frontiersman. Under .40 cal would additionally exclude you from Classic Cowboy.
Your choice of guns will be influenced by what category or shooting style you are interested in.
Calibers do NOT have to be "historic". From the book -
• Must be centerfire calibers of at least .32 caliber and no larger than .45 caliber or percussion calibers of at least .36 caliber and no larger than .45 caliber.
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Must be in a caliber commonly available in revolvers. Examples include, but are not limited to, .32-20, .32 Magnum, .357 Magnum, .38 Special, .44 Magnum, .44-40, and .45 Colt.
Although it's not common to see someone doing it, convertible cylinders in 9mm, .40 S&W, and .45 acp are perfectly within the rules, and I've done it myself on occasion. The big thing is NO JACKETED OR PLATED BULLETS - lead bullets only!
Check out the SASS wire, as recommended above, and read through the Shooters Handbook.
http://www.sassnet.com/Downloads/SASSHandbook-v18.2-2012Highlighted.pdf
Calibers, rifle types, and shotgun types required for certain categories can be quite confusing at first.
Like a lot of folks, I started with a '92, then went to toggle links (1860, 1866, 1873), then started B Western three years ago and now shoot Marlins a lot. I know a couple of top shooters in our LOCAL clubs who shoot '92's, but the 1873 and Marlin 1894 are the guns of choice for most. Just starting out, a pair of used Blackhawks, an 1894 Marlin, and a SxS would be hard to beat. New Marlin 1894's are going to be tough to find, as Remington temporarily, but indefinitely, discontinued them August 12, 2011, but used ones are out there. In August of '09, I needed a B Western rifle, and found a used 1894C .357 at a pawn shop for $249.