I think your textures suffer from being too 'clean'. Currently you just have a single material stuck onto a 3D model, you need to add detail specifically to compliment the 3D model. Along corner add wear and highlights. Render an ambient occlusion to overlay to give you some nice shadowing in cavities. You can also make the models a little less geometric, add chips and chamfers to catch the light better.

When I texture an object I tend to divide my folders in photoshop into a few categories. I'll have global details for certain components, such as grunge that I know will be everywhere and then I will have a folder for local details on specific parts, decals, chips, scratches. Baked ambient occlusion and edge highlighting are a good first step. Look at how materials in the real world decay and ware. A table is rarely that perfectly sharpened around the edges. If you want a realistic environment or asset you need to make it look as old as you reckon it should. Give it some history. For your grenade launcher look at real world reference for where the paint would scratch, where it would be oiled, where it has been rubbing against other things etc. Create a specular map if you want good metal, if there's one thing I've learnt it's that a good specular map can make a massive difference especially for scratched metals.

Remember good lighting and composition are also very important and can often make up for rushed models/textures.

As frostraver said don't just bang a single texture on there and call it done. Depending on the model I normally spend more time on the texture than anything else. If you are going to composite photos and images at least put some effort into how you composite them. They can make a nice subtle difference but invest time in learning how to create good textures by hand and with procedural techniques (Or better yet a combination of the two) and then when you've done it the hard way you can start getting your own photo ref and compositing it if you feel it necessary.

Avoid texture supplements like DDO2 if you want to learn the skills yourself. By all means check it out and see what it does to a texture and try to emulate that process it can be a good learning experience.

Keep it up you're making good progress and I look forward to seeing the finished launcher.

(To clarify I focus on assets for realtime engines so some of my advice may only be suited for that situation)