Model is a good start, but needs to be improved greatly:
First, the barrel needs to be lower poly and thicker
Second, the handle needs to be bigger and have detail instead of looking like you just extruded a box
Third, I think you need to put a hand in the model, so that it will fit on the gun correctly
Fourth, the stock needs to be a lot better, in general.
Feel free to ask me what exactly I mean by these, if you don't get it.