I just retired after 40 years in a shipyard, thirty + of those years devoted to CAD/CAM. CAD can be great for reducing time spent to produce a drawing, but it does nothing for the integrity of the design; in fact there is a good argument on the side that it can degrade the final design.
What we are producing here are replicas/reproductions of hand made items. Machines are linear, people are not. Stick with paper and pencil and don't forget to throw in some study of classic artworks. The form and decoration on Early American Longrifles all have their roots in classical art, so if you are going to build a believable longrifle you need to do your homework. Be aware however, that when you start studying classical art that you are going to run across a certain mathematical proportion that is so common that even higher mathematics assigns it the given value of phi, but if even mentioned on this board it will drive some otherwise rational people absolutely nuts.
On the other hand, you can build Southern Mountain rifles using the axiom "form follows function" and be perfectly happy until the end of your days.
One of the great things about this endeavor you have chosen is that there are so many different avenues that you can pick and choose. Just be careful not to mix too many different ingredients together in the same pot. Works great for gumbo, but for rifles; not so much.