You probably won't find a tutorial specifically on shading guns, but you can find other similar topics that could help you. One big thing is looking up how to color metallic surfaces, and another is how to shade geometric solids such as squares. A side of a square isn't,generally, going to be one solid color. It will be gradated.
I also advise using reference photos of real guns.
that's what i thought. and about using photos of real guns, it just seems very difficult to "convert" a realistic color to a "cartoonish" but still having those realistic shadings. but anyways, thanks for your reply.