Scratch building is actually so easy. Print out the blue prints 1:1, cut out the outline shape, and then trace the shape onto you material of choice, cardboard, hardboard wood, etc and cut the out. May need a few layers of that shape to get the thickness right. Glue the layers together using clamps, heavy books, yourself, whatever...
Once you have the outline the correct thickness, cut the blueprint down to just the raised areas and again trace them onto your material. Glue the raised areas in place.
Ready for sanding, bondo (optional), and paint!
Edit: Scratch building allows you to use a heavier material than card stock paper. Ideal for weapons that you will be holding.