This took around 3 to 4 days in Photoshop. I really want a lot of dirt on the hand guard so used a water rust texture and then soft light the effect and drop the fill to 20 percent. For the overall black textures I used a sandstone concrete texture and changed the fill to 10 percent then overlay then paint the black parts an off black. For the grip I just used the same water stain textures.
Lastly I added a hand print and the text COM23 and the MK14 USA for smaller details. You can download it here