Looks like you threw a mud brush on it, Makes no sense. If anything camo the sides if you're wanting a "Mud" look, or use a graphic to make it "Cool". An attempt at using realistic looking dirt doesn't really work if the dirt isn't in any place that it would actually stick to, IE where the pieces of the gun meet.
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I started with a basic texture to highlight certain parts of the rifle, and in the colour palette I have chosen for my gun.