Try reducing the vertices. A low poly weapon would need around 800-900 polygons. Also, try adding more details. I mean, not cubes, or things like that. Try using low-poly techniques such as the high poly to low poly conversions, using multires can also be a good idea. You need to use tools that will make different shapes, (if it has too many angles, it isn't very nice, if it is too round, it makes too many poly). Also, I would advice you to try adding some custom textures to make it look nicer (whatever, just use your imagination). Gl.
EDIT : remove that sphere.