Well the new barrel arrived yesterday and the precision nuts today. And when I went to make the switcher -- could NOT find the RED Loctite.... drat! Bought a tube JUST for this project and put it somewhere special....
I ended up mixing up some JBweld, and carefully glued the old barrel nut, spaced so it would headspace just perfectly right at the GO gauge, then took it off, being careful to put the nut exaxtly same spot,
headspaced the NEW bull shilen barrel, with some JBweld under the new precision nut on THAT barrel, and snugged it up also.
I do not as yet have any "flats" on the barrels, so no good way (other than rubber-coated pliers jaws) to turn the barrels, but that could be added.
I have access now to a milling machine and once I get it going well, I could mill in the flats to fit some just-about-same-size-as-barrel open ended wrench.
Not sure how open I am to TESTING it just right now, but after I get the new barrel tested out and working properly, it might be worth a try.