Rubber on the Butt wouldn't give a stable, reliable and repeatable fire position, it compresses.
As for your colours, what's important is the textures and 'sheen'
Things can be black but look different shades because they have different surface textures or level of gloss. Think about what the parts of a real weapon would be made of, a slight contrast will give the impression of anobject made up from seperate parts and components.
Just speaking from my experience as a Vehicle and Mil modeller where the only thing you hsve to make a plastic molding look like a collection of parts bolted together is to make them slightly different in their finish.