This is mine and DeepSix's Gamebanana's "Post-Apocalyptic Skinning Contest" entry!
Both of us have worked hard to produce this. The general idea for the weapon was to make something that would be easy to make with materials you can find in HL2's setting, but it would also need to shoot something that isn't a bullet, since bullets are hard to find in HL2 if you're not the Combine (at least in my book). So, I gave birth to this.
Installation instructions are pretty simple: just drop the extracted vpk into your hl2/custom directory. This skin should work with every HL2 and mods that support SteamPipe. If you want this in a mod which does not support SteamPipe (2007 and earlier), you have to extract the .vpk.
A note regarding Half-Life and any other game using the GoldSrc engine: please do not put this anywhere in there, since the skin has got dynamic textures and it will play really badly (actually it won't play at all) with the surroundings, which would effectively break the skin. So, please don't.
Preview by FlyAwayN0w: