I'll echo what he said above.
Also, in general, you'll want to have clean, crisp lines. Nothing ruins the look of a nice replica gun for me than obvious warping, or uneven lines. You'll want to pay attention to the substrate; if you're making a gun that is visually based on wood, then it needs to have a subtle wood grain. If the movie gun is metal or polycarbonate/plastic, then obviously yours needs to have a spotlessly smooth finish as well, and the paints need to be selected carefully.
Do tell us more.