First off.. give up the idea of shooting bird shot out of that rifle. With the rifling, you will not get consistant patterns, also the amount of shot per load will be so small you might be able to hunt squirels at close range, or maybe a rabbit but it really would not make for much of a shotgun. T/C has a .56 caliber smoothbore which is a small bore shotgun for doves, and small game.

As a shotgun, your Kentucky will not be of much use, and will only lead up the barrel on you. Use the rifle for what it was made for, shooting roundball. If you want a black powder shotgun, then buy a musket or smoothbore. A .62 caliber which is a 20 gauge is as small as I would go if I intended to shoot anything like turkey. A modern Knight TK2000 would be a 12 gauge you might also want to look at. You have a good rifle there, and I repeat a good rifle...