Some tips for improvements:
-Lighting: looks like you just used a (low dynamic range) environment map. There is nearly no shadow definition, maybe try to add a sun lamp with a small size to add some highlights and shadows. Or/And go for a better high dynamic range environment map (.hdr/.exr).
-Models: They’re way to low resolution, being able to see sharp corners on the edges looks not believable. Subsurface modeling is your friend with raytracing engines, polygon count doesn’t matter as much, you can go into the tens of millions without problems, especially since your using a titan, use that memory! Consider adding displacement maps instead of just relying on bumpmapping, this can look very obvious. The bullets also look way to big to fit into that barrel.
-Background: that texture looks like wallpaper, i’ve never seen wallpaper as a floor. Could also be cloth, but it’s too low resolution to tell, and is it only a simple textured plane? definately could use some work.
-Textures: Looks like your quite on the limit of texture resolution, definately use bigger textures if you want photo realism. The bag doesn’t look very much like a bag, more like a muddy and wet triangular stone, everything in your image seems to be mostly diffuse, and some objects have some sharp glossy ontop. Look at some reference photos to see how light reflects on cloth.
-Little things that matter: You can easily add realism to the cloth bag and the rope by adding particle systems to instantiate little fibers, they catch light and add a fuzzy feel to them. Maybe also experiment with different shaders setups there. Add scratches which dont reflect as much or some dirt buildup in those ridges.
-Scene Composition: find an interesting perspective, a straight on shot is boring and looks flat because you can only judge depth from reflections, also all the objects just lie there perfectly flat. No overlapping, it looks like someone carefully placed the items there. Maybe bend the rope on top of the barrel. (Curve Modifier so you can easily adjust it)
A great artwork also consists of alot more than realistic looking assets. Consider those questions: What is the purpose of your artwork? What thoughts/emotions do you want to trigger in people looking at your work? A simple background or a small scene around your objects adds alot. Imagine a shot of a pirate ships floor, flat camera angle, being able to look over planks into the sea in the background. Think of a scenario yourself.
I hope i’m not offending you with all those suggestions, you definately have a great start with the gun. I can imagine it already looking very good as a game model in the state is now. You just need to go the extra mile. Good luck!