After reading all of these posts, IMHO, take it to someone & have it bored & tap started properly. It would be one thing if you were going to do a bunch of them, but if you are going to do ONE barrel, I would take it to a place, have it done correctly, & be done with it. Instead of buying tools, rigging up multiple configurations of of holding the barrel, & using machinery that is not designed for doing the job correctly or safely, etc. Or at the bare minimum, have it drilled correctly & breech seat cut, tap started centered & then you can tap & fit the plug by hand if you wish to do so.
:idunno: Just seems like a practical approach, but everyone has dif. ways of doing things.
Keith Lisle