Pony Racer,
You would be in the Blackpowder Shootist class for GAF Muster/NCOWS.
From NCOWS By-Law 10: Rules and Regulations:
"Men’s and Women’s Blackpowder Shootist: Shooters must shoot blackpowder or approved blackpowder substitute powder in all firearms except those chambered for rimfire cartridges, and may fire handguns using a two-handed grip. Shotguns may NOT include any model of pump shotgun."
Slim