I think that would be a question suited better for the modding section of the forum:
http://forums.beyondunreal.com/forumdisplay.php?f=101
But if you wanted it to look different you'd edit the skin.
The skin is in the texture package NewSniperRifle.utx. Open that in UnrealEd, right click the skins you want to edit, then save to file. It'll save to a .dds file, and to edit that you'll need a plugin for Photoshop from nVidia's site. A Google search of "nVidia .dds Photoshop plugin" will probably find you the URL, as I don't have it handy. Edit your skin, import it back into UnrealEd, apply the texture. Don't import it to NewSniperRifle.utx, though. Import it to a new package.
Any further than this I have no idea. Coding's not my area of expertise
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I'm sure there's a tutorial for this somewhere online. Give Google some search time. Also check the UDN for more detailed info. That's probably your best bet:
http://udn.epicgames.com/