I'd make the lock first. Note, in your bottom picture, the centre line of the nipple is adjacent to the edge of the side flat of the barrel. A milling machine would be useful there to cut the nipple seat at the correct angle to the bore. Use a protractor to determine that angle and an end mill that is at least 3/8" - 1/2". A dremel tool with small grinding stones can be used to finish the nipple seat area on the plug, and files and gravers to finish the snail and the standing breech.
On Hawken castings, I always finish the plug and fence before I do any inletting. Make and attach the drip rail too, before any woodwork begins.
You have a nice lock up and joint for the plug and standing breech.
Your material is far greater than needed ... sticks out too far past the barrel...note the picture of the original from the top.