The golf ball you see is stuffed in a few cupcake cooking fabric thingies I found at walmart. There are the plain paper kind and then there are also aluminum foil ones. They fit really nicely around the ball as they are designed to fold around an object already. The foil ones could be potentially good burst discs as they are already round. They would need a little trimming perhaps.
The gollf ball is stuffed in a few of these cupcake cloths and then a small plastic cup is used to fit tightly around the whole thing, making it very slick for low friction in the barrel. I found that paper towels got ripped and hung up on the fried potato residue in the barrel, so this plastic may help. We'll see.
Also, the foam pool toy is one of those really long things, only 2 bucks each. they have a hole in the center and the whole thing could be shaves around the outside to fit in your particular barrel, and the inside hole opened up to tightly hold anything you want to shoot. The whole thing can then be sliced in half so it easily opens up once it leaves the barrel, allowing the projectile to continue on without getting hung up.
The foam looks like it would be an excellent sabot for archery arrows as well
