Another way to make mufflers... cut some large aluminum tubing to length, then carefully flatten the tube to oval shape by placing a small piece of wood on the tubing and then gently tapping with a small hammer. Don't go too far, you want to get an oval cross-section, but don't want to totally flatten the tubing. Muffler ends made of styrene sheet, drilled to accept solder pipes. Smaller diameter aluminum tube cut to short lengths and slipped over the ends of the solder pipes to create the open exhaust tips. I don't see the advantage of making the pipes by using styrene tubing with brass rod inserted into it when solder alone can be bent easily and will hold its shape just fine... yet is soft/bendable enough to allow you to fine tune the fit easily.