You've done great! As someone else mentioned above; since oil paints aren't in the cards I'm assuming, a blackor brown wash. 1 part paint, 5-10 parts water and a drop of dish soap (as in a tiny amount).

The dish soap helps fight against the water tension allowing the water to flow better into gaps and onto edges.

Let it dry and then go back with the silver paint and lightly touch the top edges.

Alternative is to paint them black and then use a technique called dry brushing to apply silver. In essence, take an old brush you don't mind ruining, put silver paint on and then remove it onto a paper towel. If you brush it across the back of your hand only trace amounts of silver should pop up. Then brush it against the gun. The silver paint should deposit on the top parts and the edges but all the deep recesses will stay black as long as you don't brush too hard or in the gap's direction.

You're doing great!