I can't tell- is the bolter's barrel hollowed out or just painted black? If it's not hollowed out, that would look better. I think you would have been better off placing highlights where you feel they should be rather than just drybrushing. I also think you can improve your whites by starting with a tan or brownish/yellowish color and highlighting up to white for a creamy look, or gray for a grayer look, or a light blue for a crisp, cold, clean look. Gray and blue can get similar effects. Happy painting