Think I figured it out
I'm in the process of building one of these. I would share pictures but am having a hard time figuring out how to upload them.
This guy has a set of 3 interesting videos on the YouTube. He even makes his own ammo from scratch.
I think Crickets build is safer and better looking but on the other hand kudos to this guy for being so industrious.
Your right madturk the link you provided is interesting.
I still cant seem to get over this idea of using water pipe for a barrel even if it is double walled. Drawn over mandrel tubing is on the market and so are used shotgun barrels. At least this guy makes his own ammo which tells me it may be light loads.