Like most projects it's best to start with an image of what you're trying to acheive. A quick scan of the web found the two images included below, the first is from Weta directly and is in higher res and therefore shows more details. The second is clip from the film which has the gun in a colour I prefer and shows it in a slightly different angle, both will be useful.
I now have a growing collection of costumes and props from the numerous fancy dress parties I've been to. This gun is rather large and I don't like the idea of moving it from house to house as we bounce around so I have decided to ebay it. If you read these instructions and decide that you would like to own this gun now is your opportunity....
This and all my other projects can now also be found on my own website at http://msraynsford.googlepages.com