Pulp, SASS#28319 here. My favorite sport. And I'm the World's Worst Cowboy Action Shooter.
Now to your question. Unfortunately, you have to have a rifle in pistol caliber, it can be a lever action or a pump, and you have to have a shotgun, 20, 16, 12 or 10. SXS, Win. '97 or clone, or Win. '87 or clone thereof.
There is a category for percussion competitors, called Frontiersman. BP or sub must be used in long guns as well as revolvers. SXS or '87, no '97's allowed. If a stage calls for a reload, you charge all 6 chambers, but only cap five. Before you fire the first shot from that revolver you cap the 6th nipple. I usually shoot Frontiersman category.
On the plus side, you should be able to pick up a used Rossi '92 for under $350.00 and a decent SXS for around the same price.