Dude.
I downloaded and tried to animate it for you, but bumped into a bit of a problem. Importing into milkshape took 10minutes!
The mesh is over 100,000 tris.
Have you any idea how huge that is?!?!
That's like 33 colonel X's
The flat face on the shoulder butt of the gun is 8000 tris. It could be 3.
I'm gonna waggle a slightly accusing finger and say, did you make this? And if you did, did you get a bit excited by the smooth geometry button?
I'm sorry to be so harsh, the model is cool, the final result looks good, and the texture is passable, it's just that you have made it way too high poly.