What is the smallest diameter octagon barrel I can get in .20ga. I am ready to order parts for a new rifle and want to build a large caliber rifle with an extra .20ga. barrel. It will be a half stock, at least .54, using a late Ketland flintlock, and a really nice piece of maple. I tell myself that my ancestors had Tn. mountain rifles as they moved west to Louisiana and at some point had them cut down. I have made 3 of them and they are great woods guns but this one I want with an extra smoothbore barrel. Danny