Change your material's diffuse to ~40% and up the spec, and set roughness to 100%. Makes it easier to read. I hope you're watching (or have watched) Milennia's excellent tutorial on this weapon? It looks like you have some edges that are too sharp; specifically the chamber and the ends of the barrel.
You've fallen back to having the brighter diffuse material which makes the specular harder to see (which makes comments on sub-d errors less likely). Regarding the lowpoly for that, you might find something useful in this thread: http://www.polycount.com/forum/showthread.php?t=124852 To get the backfaces, did you try…