People have been making paper templates for a long time, I can't see how plastic would have any real advantage. A plastic guide isn't going to constrain a metal cutting tool, at best it just shows you where you need to drill the same as a paper template. If you wander outside the lines you'll just mess up both the part and the jig.

If I were to set up a clandestine gun manufacurer I would try and design a product that could be made using mostly aluminum extrutions and paper jigs. That way it's easy to compartmentalize each step, harder for one guy to flip on you, and fast/cheap. Plus if you get raided you don't have a bunch of incriminating files cached on your CNC machine from previous runs.