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
Step 1: The materials
XPS foam, this is the pink loft insulation foam and is used to form the base of the gun.
Isopon P38 (The easy sand stuff), used to give a solid plastic coat to the weapon.
40mm plastic pipe, used as the core to the nobbly bits
8mm threaded rod (or similar solid bar) to connect the nobbles to the rest of the gun
Bottle caps, used to plug the pastic pipe
5mm dowel, details on the nobbles
Cardboard, used for details and tubes
Brown paper (any paper will do) used for the paper mache parts
PVA Glue, used as a protective cover to stop the solvents eating the foam
Hot glue, used to stick the foam parts together
Black and White acrylic paint. Many shades of grey to be made here, one of the joys of black and white weapons. Gun Metal coloured acrylic for highlights and weathering