I have no idea what minimum thread size would be, that's why I went maximum. I mated up surplus .50 cal barrels to the stub with 7/8-14 thread. I also used a 5/8" dia. breech plug approx 2" long that was simply a slip in fit, using an "O" ring and a tapered nose for sealing out gases. It had a built in extractor that laid over the shotgun extractor to eject the 209 primers when the breech was opened. I drilled and tapped a 1/4-20 hole on the left side of the barrel and used a dog pointed set screw to keep the breech plug from rotation and ejecting with the primer.
If I were to build one for smokeless powder, I would make the barrel solid all the way back and tig weld the lug to it, like the factory Handi-rifle barrels.