Katherine Dewey published a good, step-by-step book on sculpting human anatomy with polymer clays (Super Sculpey, Fimo, etc.) that I read recently and would recommend for aspiring sculptors. Nothing in it on making guns though.
think of it as a camera that slowly comes into focus. Just start with a rough shape, then slowly refine it bit by bit. Mark Alfrey doesn some good video tutorials, should check him out.