Let's say hypothetically you have CAD designed a pistol frame and slide that uses an off the shelf barrel and trigger group. A 3D model would be printed in house to verify basic fit and function. Lacking the necessary CNC mill, is it legal to have an outside machine shop CNC the frame and slide? If not, is there another way to do it? Not looking for end-arounds, looking for the proper and legal way to do it if possible. TIA.