just get uvmapper classic (it's free) read some tutorials in the website, play with it, you will get the hang of it.
UV mapping is not a perfect science, just imagine like you are gonna slice your mesh (3d model) in pieces, then you grab each piece and unwrap it the best way you can, just pick new uv and select the one that looks the most like your piece of 3d mesh that you are going to unwrap. there are several that you can choose from, planar, box, cylindrical, cylindrical cap, spherical. For example if you are going to unwrap the gun barrel you could try cylindrical or box and also the point of view that you will use to unwrap it. Like I said, is not a perfect science, is like a trial and error thing, then rezise it and put it in a corner or something, then go to the next piece.
there are good tutorials around. You can search here in the forum.
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