If you're mapping your gun from the side, as that image would suggest, have you tried downloading some photos of guns, piece a few together to make them fit the weapon shape you've made, then mapping it to your model.
Sometimes a photo can be mapped straight on and work really well, especially on an object that's thin in one plane, as a gun is vertically, so you don't have to worry about the texture stretching across the top and bottom polys.
Sometimes a photo can be mapped straight on and work really well, especially on an object that's thin in one plane, as a gun is vertically, so you don't have to worry about the texture stretching across the top and bottom polys.