I couldn't find a suitable size plunger so I improvised and used the lid from a toddler's drinking bottle. I spray painted this black and used two pieces of aluminum tube for the plunger arm.
I made the gun from piece of wooden dowel, spray painted to look metallic. I used a 1/16" drill bit to make holes for the gun rods to fit into. The rods are made from piano wire that I managed to get from a local hobby store.