The gun arm is a length of PVC pipe painted silver. I used the bottom straight parts of wire coat hangers (thanks for those Joe!) for the 8 rods that go around the barrel. I was able to bend them into the correct shape with a pair of pliers.
For the plunger arm, I used 2 lengths of PVC pipe that would slide into eachother and then ran a broom handle through them with the plunger cup screwed onto the end. I got it to telescope by gluing a PVC ring to the broom handle about 2/3 the way down so it would push the middle part of the arm out when the arm is extened. That's probably not exactly how you're supposed to do it, but it works.
The ball joints were Magic 8 Balls that I drained of their advice-giving (and finger staining) blue liquid, and drilled big holes in. (See post on that here.)
Here are the completed gun and plunger arms:
