Hm... wait. I didn't see your high poly before. Looking at your high poly... it's so radically different, it will be very difficult to fit a texture over it without rendering out an ambient occlusion map based off the high poly model first.
That means, I can't modify your unwrap at all.
Unless I'm also willing to do the high poly to low poly baking myself. Which, I'm not, sorry.
The process is still new to me and it's very intensive. I have actually only done it once before. For this gun:
The difference is. The model above was actually a high poly baked unto a
medium poly. Very little difference between the two (just smoother edges), so I actually could texture it from the lowpoly itself easily.
Your model is more problematic because there is a sharp difference between the high poly and the low poly which
necessitates the texturing to be AFTER rendering out the normal map and the ambient occlusion map (for texturing reference, as well as picking out shadows of the grooves and whatnot on the highpoly).
Also, once the high poly has been baked unto the normal map and AO of the low poly, it will then require very little further texturing.
So... I can't do anything with it until you have rendered out a normal map and AO, sorry.
P.S. Also I suggest reunwrapping it to minimize the empty spaces between the islands as much as possible. Your normal mapped low poly will look very bad if the resolution is too low on the texture itself. So it's in your best interests to use as much of the space in the UV map as possible so your normal map remains sharp and not smudgy or blurry.
Indeed, if you can, I suggest halving the entire gun then unwrapping it as it would mean you have less surface area to fit into the map. Then halving the high poly as well then baking its normal map to the halved low poly. Then mirror everything again. This will result in a much better quality normal map.
Also for unwrapping, it is helpful to use a checkered material when unwrapping your model. And to try and keep the squares from distorting. The blue checkered area are the areas I had started to reunwrap. The blue squares are not distorted unlike in your unwrap (the black and white textured one). For instance. Try applying a texture with lettering on it unto your model and you will notice that the letters are very distorted, the distortion would actually be impossible to correct in texture.
Don't be afraid to not keep the vertices together in the uvmap like in the bottom picture. That can be compensated via texture as long as the seams are properly aligned. It's much better than distorting your uv's in the interest of keeping your islands together.
Remember that this is not an organic model. Organic models can afford that, but hard surface models like guns need to minimize distortion as much as possible.
Anyway, as I said. It's very intensive, so... I'm sorry I can't do this I guess. :/ I mean I can, but I really don't want to spend 2 weeks on it. Heh.
P.P.S. Use Box/cylinder autounwrap as much as possible. 3dsMax has the Box mapping tool. Box map it first then snap and adjust the pieces generated together. I don't know about blender and other apps though, but I'm pretty sure they have that too.
