I have a formula and "order of procedure" for making a full scale drawing that, with actual parts to measure and or trace, given stock measurements and the afore mentioned known locations and relative angles it isn't too complicated to make a decent drawing. I started doing this three decades ago for muzzleloading rifles that were much more complicated than most breechloaders.

As promised, here is a better pic of the Martini barreled 6.5 Hagn action: