You could make everything on a gun out of plastic (commonly done), except for the parts subject to the detonation and bullet (chamber, barrel). Those COULD be made from ceramic. A plastic projectile would also be possible (it'd need to be pretty dense; not sure how best to do that -- it would still look like a bullet on an x-ray, unless inside something else, in which case you might as well do lead inside a form-fitting lead case, inside another object)