Quick and dirty edit to explain a few things-



First of all, the line art needs a bit of work, especially for function. Remember that the grip and trigger area is based around a hand, so it needs smooth, ergonomic curves and a decent amount of area for your fingers. The way the whole thing is designed now, with all the little rounded bits and bumps everywhere, makes it look a little lumpy. You can still keep details and style without having to break the silhouette, and sometimes (see: most of the time) less is more and all that.

Now, as for the shading, guns are very simple and geometric, so you need to be very deliberate. Try to avoid random clusters and try to keep the major geometric forms (look especially at the grip, it was shaded as three separate areas when they all form the same cylinderish shape) If a part of the gun would just be a flat surface, don't be afraid to leave it alone with just one shade. Again, less is more.

And with the colors, one of the first things anyone learns about metal is that it's high contrast. Get those dark tones really dark and heavy and get the light ones bright and reflective. I also simplified it a bit since having red, green, blue, and yellow all together is a little crowded and crayon-y if that makes sense. The blue worked better with the read and yellow so I spread that around and that's that.

Also try to avoid any dithering on something that's supposed to be smooth. It can work, but it takes a lot of skill and discipline.