Been there, done that.
“Mr. Single Shot’s Book of Rifle Plans — designed for the serious gunbuilding enthusiast. Included are detailed instructions on how to build four unique breech loading single shot rifles, from metal stock to a finished rifle.”
Already own that book.... and many more.
I think the first one I make from scratch will be a 1878 Borchardt or a 1885 High Wall.
What I’d like to find is the right sized shaper to do the interior corners and outside flats. Shapers leave a nice, clean surface finish from their cut, unlike mill cuts.