Show me your UV unwrap. If you are just wrapping that bitmap via front view, I have nothing more to say to you.
As for the mesh itself, you need to work on vertex density. Ignoring the fact that this is too heavy for GunZ, if I was planning on putting this in a game, I would worry primarily on what happens when I increase the vertex count for high poly uses. Whether that be for first person view where higher detail is needed, or simply to pull some bakes off to make some AOs, there is no way some of these vertex loops would subdivide nicely. When doing initial creation, spread the mesh data out equally, then as you ad detail there will be less difference between how much information is kept in each location.
