Not really an update, since I haven't done anything yet, but I did find my notes: "Checked bbl shoulder w/ tenth indicator, all OK." (Yeah, that's a .0001 B&S)
"Scraped bbl channel. No contact except chamber area."
"Checked bbl to rec. ring w/plastiguage; no gaps."
"Still shoots left."
I've moved since then and other stuff has gotten in the way, but the notes jogged my memory -such as it is. So I do believe its a bbl issue. The whole rifle weighs in at about 12 lbs. Most of that is bbl.
I'll bother you some more when I learn a little more.