I would make up a variety of component parts - tubes, shaped boxes etc. and glue them together before laminating over the top.  You ought to be able to make it look quite realistic.
If he intends to handle them a lot, you may have to include some packing or make the laminate very strong.
DavidO

At the scale your are planning to work with I would be inclined to get a piece of wire and bend it to shape, although it is difficult to bend coat hanger wire is good and holds it's shape.  Bend first then cut to length.  laminate a couple of layers of paper around the wire. (newspaper is good, kitchen towel is ok but avoid tissue paper as it breaks up too easily.) When this is dry use the fine finish pulp to shape the barrel and stock of the gun, once the pm is thoroughly dry use jewellers files to sand down and shape the piece. 

I have used this technique for slender and detailed pm, my dolls legs and arms are made using this method.  I would aim to paint fine detail rather than pm for it.
Alternatively use Sculpey, as you will be able to model this in this scale slightly more successfully.

Jewellers files are small metal files used for removing burrs from the surface of jewellry during the silver smithing process.  They are fantastically useful in PM as they are small and can be used to file down difficult to reach areas.  They are sometimes referred to as key files or needle files

http://www.jewel-toolcraft.co.uk/Files- … B2VCS.aspx

Hope that helps.