Did you make a high poly of this gun first? Or is this only the blockout? I'm also a little confused about the back part. Is the design actually faceted like that or is it supposed to be perfectly round and you're just trying to stay within a polygon budget? The front has a much smoother silhouette than the backside and it…
I don't think it's so much a matter of increasing the polycount as optimizing it. Take some of the polygons you've spent on small or inconsequential areas and apply them to larger or more noticeable ones. All of your spheres, for example, seem to have the same number of triangles; those tiny ones at the end of the muzzle…