I would certainly think the chamber must be cut from your barrel. Its safety is at least partially determined by how thick the barrel is at the front of the chamber. I have made a caplock shotgun from a modern barrel, but I bought one un-chambered from Numrich corporation. It was very thick in the breech area so it could be turned to fit many different guns. What you are proposing has many potential problems including the outside diameter fitting existing stock. the bolster/nipple area if it is a caplock and flat side with vent if flintlock. The remaining barrel thickness at the breech end after it is filed to fit the stock, the under rib and thimbles and such and so fourth. If I were going to do that, I would start with an octagon that would fit the stock, and have it bored put to the desired shotgun gauge, after I determined what kind of breechplug would fit my gun. It would be easier to simply make a shotgun, me thinks. Note: I am an amateur and not officially any sorto f expert.