In a hurry so i apologise if this sounds blunt; - You resized the stock unproportionally, it's a lot wider than tall - That grass brush is a big no no - The gradient you overlayed is a good idea but a bit extreme, you only want to be tweaking the blue background, not creating a white / blue / black one - The Spartan is a bit blurry, try duplicating the layer, sharpening it once or twice and putting it on 50% opaciety, and then press CTRL+E to merge them together - The text shouldn't be orange. You should be using a text colour that compliments the sig, try to use colours already present and you'll find the text blends much nicer.
That's what I'm hearing from others too :/ ... but you can't really tell the difference when you just look at the sig, instead of putting the stock and the sig right next to each other. Oh, I tried matching the text with the orange in the helmet.