It has a 1" x 12" PVC stock/chamber. The main valve is just a modded sprinkler and the trigger is a blowgun.
To be able to use the shells, I glued a 1" x 1/2" bushing into a 1" male adapter. This piece is the heart of the shotgun. It connect to the sprinkler and provides a socket (1/2") to plug the shells into.
The glue is still curing... but here is a picture. I also took a video of the shells being fired with a different sprinkler valve gun.



Reloading the shells is a breeze. You just pop off one cap and stick a cotton ball in the end of the tube. Replace that cap and take off the other one. Fill the tube with bb's leaving about a 1/2" of tube empty. Stick another cotton ball in and put that cap back on. You have to take the caps off when you're shooting but I like to keep them on while I'm carrying the shells just to make sure nothing falls out. They are very secure though without the caps.