I think the painting and composition is fine, the thing that bothers me is the over all look. It looks more like a super soaker than an actual gun.
I'm not sure if you did some research or used a reference, but if you didn't you should have. I suggest looking up various references of certain parts of the gun.
Example:
For the pistol grip, look up pictures of real pistols, grips, and concept pistols, grips. And even in games. Do as much research as you can on the part you are drawing and do this for each part of the gun.
You will get an over all better and more realistic looking concept then just thinking of something off the top of your head.
Also, if i were you i would consider how you would hold the weapon, and the actually usability of it. Take the handle/pistol grip of the gun. It looks like it would be very awkward to hold, therefore it losses some realism, because real gun makers or weapon smiths wouldn't make a weapon that is awkward to hold.
Same goes for the shoulder rest, it would look more comfortable if it was more of the contour of a shoulder than a flat block.
I also think it would be a good idea sketching the gun from multiple angels to give the viewer a better idea of the gun along with the finished piece like it is now.
All and all, your painting skills are unique and i really like them. But you should take in mind the things you draw and always double check to make sure they are correct in the real world if you are gunning for realism.