Well, if having a short piece of tubing in my shop (which might potentially, someday, become a shotgun barrel) is illegal, I'm going to need a stack of tax forms and a deep pocket-book. I've got lots of old tubing pieces out there that I've stubbed from longer pipes. There may even be a piece of old barrel in the scrap bin. None of them have chambers in them, but if I correctly recall my youthful indiscretions, I do believe that a 20 gauge shotgun shell will fit nicely into a piece of 3/4 galvanized water pipe. I"ve been known to use that for cheater pipe, so I've got potentially a half-dozen short barreled shotguns in my tool chest.
I guess that the question would fall on the definition of 'rifle barrel". When does a piece of cylindrical metal become a rifle barrel?