I'd focus on your high poly modelling and baking before diving into Substance. Your dirt doesn't seem to conform to any shapes on the object at all, and the tape (leather? it's hard to tell) wrapped round the stocks looks like it's melting into the wood and metal, rather than a separate object like it would in reality.
The butt of the gun also looks more like rotting driftwood than machined wood.
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The butt of the gun also looks more like rotting driftwood than machined wood.