Adding to found objects (toy guns, etc)

I have two toy guns I want to add on to. One looks great but the barrel needs to be thicker, so I want to add a length of PVC over the given barrel and fill the space between them with putty or similar. I could also use to fill the inside space for weight and fill screw holes. What would suggest that would be easy to work with and cheap?

Plumbers Putty ? Have some of it I have used for real projects around the house and have seen others use it for some prop work.
i would add use all precautions when using it (gloves and other precautions). It hardens and binds really well and has a good weight to it.

Be sure to post what you use and a pic !

Any 2 part epoxy putty will work.
As mentioned Plumbers Putty (don't get the marine or steel types)
Aves, Magic Sculpt.

For filling between the 2 tubes use shims.
Plastic, wood, what ever fits.