what splosh999 said
The rendering is very well done and most of the proportions are too. However, it's the posing in the arms and hands that throws everything off.
If you tried to hold something solid like that gun behind your shoulders, it wouldn't be possible, let alone comfortable. A person's body just doesn't unhinge like that.
If you were to bring the gun behind his neck though and adjust the arms according to that change, I think this could have looked much better. The hands also look very uncomfortable at the angle they're in.