It's never easy to jump right in and design a gun - personally I always find it quite easy to recreate guns in comparison. Like get some good picture references, and a nice big side on image, then tracing around that with polylines and extruding gives a great starting point. Using the side on image as a texture too cuts out most of the work.
The detail you put in really depends on what sort of game your aiming for, like a FPS game tends to only need a very basic handle, and often with the most concentrated detail on the visible side of the gun (like safety levers and bullet eject parts).
It's not too bad for a first effort, but I'd suggest trying to recreate stuff first, it's quite satisying to recreate the mechanics as well, so model the clip and make the trigger movable - again that depends on what it's for, everyone likes realism in guns though.
Van-B
Put away, those fiery biscuits!