This is a very interesting project! I myself have two on the go at this moment(1 in .500" NE, 1 in .577" NE) looking at the start point that you had,it would probably have been easier to start with a block ofsteel,as I have done,the first thing that I did was to wire cut the breechblock mortise down through the billet of steel,everything is then built around that hole,as the saying goes,"whatever you do in life,you always start with a hole!" I really did not like the idea of welding bits on,so I even machined the long top tang out of the billet,it does require the barrel thread to be screwcut before it is "bent" to shape though. Looking at your photos has given me inspiration to get back on to finishing my two !!
Chris.