My friend Bruce Yeany has made a pop gun video with a good idea about the sealing material.
I got a nice email from Sonya in Pennsylvania, interesting and helpful feedback:
"Hello! I wanted to let you know my son and I made a pop gun using a combination of your instructions and the ones in the video. It came out great! Just a couple things I wanted to mention as they worked out well for us - we used some foam from old floor foam tiles (the kind you use so little ones don't bonk their heads on hardwood floors) for the seal that we screwed onto the dowel. Also, we used the method from the video of using an eye screw in the dowel. This made it very easy to tie on the cork - we used thin, round shoelace for the repeater string as our hemp cord kept breaking after about ten shots. We were able to find cork in the specialty hardware aisle at Home Depot (in a giant drawer that I would not have seen if I hadn't asked). For a handle we used a wooden toy kitchen knife tied on to the dowel."
You can make this loud version of the classic even if you don't have a cork.
There is a lot of great science in a popgun. A person building and using on has to grapple with atmospheric pressure and sonic compression waves.