It's decent, but as the people before me have said, work on your shading. It's rather jagged and rough right now, and it doesn't give the impression of a single lightsource. I won't say that one lightsource is the only correct thing to work with, but it certainly is easier than multiple. Also, keep in mind that a lightsource coming from above looks much more natural than one coming from below.
You should also practice doing outlines, as these should never exceed one pixel of thickness, unless you're deliberately going for a thicker outline, but it should always be uniform. Your outline is also a bit messy, in that it isn't symmetrical where it should be so.
Also, some of the colors you've used are abundant. If you can barely see the difference at x10 magnification, you won't notice it at all at full view, and can therefore be removed. Also, be careful with those near-white colors, as they tend to stand out A LOT.
And as a final note, triggers on guns tend to point outward, so your finger fits into the curve, and the area around the trigger is... Well, air. Except for the finger/trigger guard, which should be a bit thicker than the outline.
For some unexplainable reason, i've felt the urge to show you how it COULD be done.
I only changed the outline and shading, but notice how i did it. The non-straight sections of outline has had the so-called "jaggies" removed, and i've reshaded the gun to show a lightsource in the top-left corner. Also, the trigger area has been made a bit more realistic, though i don't know if this is what you're going for.
This is all just a great big suggestion, and as anybody would tell you, what i'm doing isn't necessarily the right way.