What Mike said... Or, another method that can be used is an opaque projector. If you have a drawing or photo of a particular rifle, enlarge the image onto a wall with the projector. RCA I&II (and other books) give the overall length so you keep everything in proportion. Tape a big piece of paper to the wall and trace the projected image once you've enlarged it to size. You'll need to have some measurements of the original to do this, but it works well.
Ed