a suggestion: the dof is a bit too much. the area where it is sharp is a bit too small. maybe you can change it so that the beginning of the gun is just out of focus, rest of tank in focus, and the background right behind tank not in focus again. another thing i noticed, the ground is too flat for such a dark earth. maybe it would be better looking with desert - but even desert is not completely flat.
other than that, great model and texture on the tank