Try fireing the gun , if it has an effect on what you shot it at, and the character animation seems otherwise normal,then while in third part view move the camera aroun and see if you can spot what will look like a rediculously tiny version of the wepon.
If you see that, usually save- exit- reload game reload save will usually clear the problem, what seems to cause the problem is a scaling glitch from switching the same model from workstation veiew to ingame view.
repost this on the bug forum so the devs know about it.