If I was starting with a rectangular block, the first thing I would do is to check out the grain flow of the wood.
If there is any angle to it at all, I would want that running parallel with the wrist. This would tell me which end is going to be the top and the butt.
I would then draw a rough outline of the finished shape leaving at least about 1/4 around where I wanted to end up.
After finding the top and the butt, I would take it to a wood working shop and have them cut it down so the finished thickness is about 3/8 to 1/2 inch thicker than the thickest area of the finished gun and plane the top and both sides so they are flat and square.(I don't have any precision wood working tools). The bottom isn't real important because it's going to get cut off anyway. I would just make sure I had left enough wood on it to have wood to remove for the toe plate at the butt end.
I would then draw the finished outline on one side. This doesn't have to be exact because it is only going to tell me where the breech will be from the muzzle end and the butt end. (These ends need to have at least 1/2 inch of extra wood because this whole process is not precision and I don't want to end up without enough wood to install the nose cap or the butt plate.)
Knowing this, I would then cut the barrel channel into the top surface, right down the middle, stopping just short of where the breech will be.
This cut would be the full depth of the barrel, if not 1/8 inch deeper. (When the barrel is inserted into the channel, it should be flush or better yet below the remaining top surface of the block.) It would be cut so the centerline of the bore is running parallel with the top surface.
After squaring up the breech surface so it mates with the breech block or the rear of the barrel, I would then start to determine where the lock will have to be based on the location of the breech plug face (in the barrel).
What I would do next would depend on whether the lock/plug was precussion and snail, or precussion and drum or flintlock but before worrying too much about the lock, I would install the breech plug and inlet it into the wood.
Notice, I am leaving the sides parallel and square with the top. This is to facilitate measuring for future things like barrel pins etc. It's a lot easier to draw on and make the layout lines for things like ramrods etc if it's flat.
When I inlet the lock plate, the barrel will determine the depth and I can assure you the face of the lockplate will be well below the surface of the side of the block. You can always remove wood to get it down flush with the lockplate. It's dam hard to put it back on if the plate ends up standing proud of the surface.
Well, that should keep you making chips for a while! ::
Anyone see any screw ups in my method yet? ::